The Great Explainers – Carl Sagan


My first proper blog post.  Whilst perhaps later, I’ll get my blogging shoes dirty with something substantial, for the moment I’m going to keep this fairly light.

Mark Esner, Adam Boxer, Ben Newmark and HistoryLover have recently written on “The Great Explainers” a series of blog posts on what makes a good explainer and explanation.  They have stimulated me to finally get started and blog on my favourite explainer, Carl Sagan, and what makes his explanations so perfect and so memorable.

Sagan* embodies many of the features that have been mentioned by others in this series of blog posts.  His explanations are delivered with the confidence of an expert, for indeed he is; his qualifications, accolades and day-job leave you with no reason to question his authority on any matter. He delivers his explanations with a precise language, his words carefully chosen to be rich and interesting, and yet clear and precise. He takes time to build his explanation carefully in easy to understand steps.

But what Sagan adds to his explanations, is an emotion, warmth and humanity. He does this by telling the human stories of discoveries. A classic example is his telling of the story of Eratosthenes, an ancient Egyptian polymath who read in the library of Alexandria of a site in Syene where columns cast no shadow at midday on the longest day of the year, how intrigued by this (since at the same time in Alexandria, columns still cast a shadow), he paid a man to pace out the 800 km to Syene and then used some simple Pythagorean geometry to calculate the curvature of the earth between Alexandria and Syene and therefore the circumference of the planet. Over 2000 years ago (take note Flat Earthers).


Sagan’s explanations are particularly memorable because of the extraneous human details of the story. We are, after all, evolved to remember human stories.

Primitive humans were at an evolutionary advantage if they could listen to, understand and remember a human story.  There was a benefit for our ancestors to listen to the survivor-story of a their fellow cave man; returned from a days foraging having narrowly avoided a wild animal attack.  By being able to recall the details of the story; the sounds of the bear and location of the attack as described by the surviving hominid, our ancestor avoided getting eaten by the hungry grizzly, whilst those who twiddled their thumbs and talked to the person sat next to them were doomed to be the next ursine meal. And thus, the genes for carefully listening to and remembering human stories were passed on and strengthened in humanity, and we became a species of story tellers, passing on our oral history from one generation to the next.

When I recall all my favourite lessons, lectures and demonstrations from my education, the most memorable ones always included details of the human stories behind the discovery.  For example, I vividly remember university lectures on the nerve action potential, which told the story of their discovery by Hodgkin and Huxley working on squid giant axons at the laboratory of the Marine Biology Association in Plymouth.  Their story was told to me, with the humanity included: their struggles to find a suitable nerve preparation in Cambridge that was large enough to take a micro-electrode; the pausing of their work for 8 years during the second world war; Huxley’s work on Radar and anti-aircraft guns during his war secondment; the destruction by a German bomb of their electrophysiological equipment in Plymouth; and most vividly of all the fact that post war, squid giant nerve axons that had generated good results would be “immortalised” by being thrown violently up, over their shoulder to stick to the ceiling, a practice that was continued by subsequent electrophysiologists working in the same laboratory.  Not only did I never forget the accompanying explanations of the nature of action potentials, but so fascinated by the story was I that I spent the next 10 years studying electrophysiology and would name my son after one of those great men.  Indeed, I got to see for myself the spagetti-like ceiling pattern of dried cephalopod nerves, still preserved in situ, when I visited the Plymouth laboratory during my PhD in 2002.

Hodgkin Huxley

Huxley and Hodgkin with their electrophysiological equipment.  From the cover of the Nobel Prize Programme Cover in 1963.

There are some that might argue that these details of the human experience are unnecessary and clutter the explanation of the required detail. But I would argue that the human stories are what make our brains sit up, tune in and take notice of the details. They are the hat stands on which we hang the rain coats and umbrellas of knowledge.  Without the human stories our minds are left to sort through the jumble pile of wet coats, often getting bored and giving up.

In almost any online video of Sagan you care to find, he always finds the human element to his explanations. His explanation of 4-dimensional tesseracts begins by asking us to imagine we are 2-dimensional beings living in flat land.  Or his answer to a young girl’s question “Is the sun part of the milky way galaxy?”

You are part of the milky way galaxy”. He tells her, the look on her face tells you she wont forget the answer.

So natural and brilliant is Sagan’s ability to tell a human story that he wrote a fantastic science-fiction novel that is scientifically plausible about the search for extra-terrestrial life in the stars, and what might happen if we were to find such a message. The book is Contact, and was made into a Hollywood blockbuster of the same name starring Jodie Foster.

In my opinion Sagan’s only peer in the ability to find the humane and emotional in any dry discovery or explanation is Bill Bryson who similarly tells so many human stories in his incredible Short History of Nearly Everything that the book is neither short, nor put-downable. It is the only book I can honestly say I’ve read completely from cover to cover more than twice**.  Yet while Bryson might own a roll-neck sweater or two, he doesn’t have that luxurious, imitable voice.

Whilst I don’t try to imitate his voice (often) in my own teaching, I do try to find the human stories to accompany the scientific knowledge I am imparting each lesson.  It’s why I love reading lay science books about scientific discoveries which I can then pass on to my students in the oral tradition. I find it gives everyone somewhere to hang their hats.




*my awestruck, reverence means I can never quite bring myself to call him by the familiar, Carl

** if you exclude the favourite bedtime books of Huxley and I.


Materials From The Earth – Part VII – Rock Cycle

Rock Cylcle

Today’s learning objectives:

  1. To understand how fossils are formed
  2. The understand how sedimentary rocks are formed and may contain fossils
  3. Understand how rocks turn from one type to another in the Rock Cycle.

Task 1

Make sure that you are caught up on the last lesson’s tasks. You should:

  • have written a description of how sedimentary rocks (like sandstone) are formed (from sand).
  • have written a short description of how fossils are formed in sedimentary rock. If you need to you can watch this video again.

Task 2.

Watch this video on the rock cycle. You can also find information about the rock cycle on the BBC Bitesize website and the Geological Society Website for Schools.

2.1 – Draw a diagram of the rock cycle, explaining how rock types turn into one another.

Materials From The Earth – Part VI – Sedimentary Rocks & Fossils

Lesson Objectives – by the end of this lesson you will:

  1. understand how sediments get stuck together to form sedimentary rocks
  2. understand how sandstone and limestone are formed
  3. know that fossils are the shapes of dead organisms preserved in sedimentary rocks


There are 3 tasks for today’s lesson, you will complete them all in a circus, one at a time.

Task 1 – Sand & Sandstone

girl experimentwriting task

This is an investigation and writing task.

You will have a tray of damp sand, do the following things, observe what happens and write down your findings:

1. Observe the sand with a hand lens. What structure does the sand have?

2. Take a handful of sand and squeeze it as hard as you can.  What happens?

3. Carefully examine the squeezed sand. What has happened to it?

4. Can you see a shiny coating on the grains of sand?  These are mineral deposits that were dissolved in the water and are left behind when the water is squeezed out. Eventually these mineral deposits would cement the grains of sand together.  What would happen to the grains of sand if they were stuck together?

5. We can experiment with different types of cement by mixing sand with a) plaster of paris, b) clay – see what happens to pellets of sand mixed with different cement.

6. Take a look at your pet rock – is it a sedimentary rock, how big are the grains?


Task 2 – Different Sedimentary Rocks

investigatewriting task


This is an internet research task.

There are many different types of sedimentary rocks, you are going to do some research about some of them. Draw the table below in your books and use the The Geological Society website for schools to help fill in the information.   Does any of this information help you identify your pet rock?

sedimentary table


Task 3. How are fossils formed?

investigate writing taskdrawing


This is a research, writing and drawing task.

1. Watch this video, about how dinosaur fossils are formed in sedimentary rocks.

2. Explain in your own words how mold and cast fossils are formed.

3. Using your understanding of how igneous and sedimentary rocks are formed, can you explain why don’t we find fossils in igneous rocks?

4. What type of rock is your pet rock? Does your pet rock contain a fossil? How do you know? Could it contain a fossil?

5. Take a look at some of the fossils we have in class. Draw a sketch of your favorite fossil.


6. Extension – do some research on your favourite fossil – what animal is it? How long ago did it live?   Some ammonites (see picture above) lived 100 million years ago, that is older than the dinosaurs!



Materials From The Earth – Part V – Weathering & Moving

Lesson Objectives – by the end of this lesson you will understand how:

  1. water and sunshine can break rocks apart.
  2. plant roots can break rocks apart.
  3. rocks are carried and broken by moving water.



Weathering is the word given to the breaking and changing of rocks by natural processes.  We learned about one of those processes in Part IV, acid rain.  Today we are going to learn about other types of weathering:


Task 1.

investigatewriting taskdrawing

This is a personal research task, also a writing or drawing task.

Below is some information, and resources (videos and web-pages) about different types of weathering.

1.1 Read the information below, then either:

a) Write a description of what happens in each type of weathering in your book, this could be under headings, or as a table.


b) draw a poster with pictures/cartoons which describe the 4 types of weathering.

There are 4 main types in all, they are:

1. Onion-skin weathering.

onion-skin weathering

onion-skin weathering

This is where changes in temperature (hot during the day and cold at night), cause the rock to crack and break off in layers.  This is common in countries where it is very hot during the day and very cold at night, such as desert countries.

Can you guess from the picture why it’s called “onion-skin” weathering?

2. Freeze-Thaw Weathering.

cracked rock from freeze-thaw

cracked rock from freeze-thaw

This is more common in the UK, where we don’t have that much heat during the day, but we do have lots of rain and it often gets cold enough for water to freeze. When water freezes it expands. If there is a crack in a rock full of water, then the freezing water will make that crack bigger.  Then it thaws out and fills with water again, before freezing and making the crack bigger. This process continues until the rock breaks in half.

You can find out more about freeze-thaw weathering on this video, this video, or on the BBC bitesize website.

3. Acid Rain Weathering.

Acid Rain wore away this statue on Bath Abbey

Acid Rain wore away this statue on Bath Abbey

In the last lesson you tested rocks for weather chemical weathering by acid rain affected them, so you know now how acid rain effects some types of rocks, and buildings and statues made of those types of rocks.

You can read more about acid rain on BBC Bitesize

4. Biological Weathering.

A Tree Growing in a Rock

A Tree Growing in a Rock

Biological weathering is where rocks are slowly broken apart by growing trees and other plants. The roots, in searching for moisture grow into cracks and as the roots grow the cracks get bigger. You may have seen this for yourself on a small scale, where paving slabs on the pavement or on patios are moved and broken by growing trees and plants. There are some impressive pictures of biological weathering here.

Erosion, Abrasion and Transportation. 

Watch this video which shows the connection between weathering and erosion by rivers and glaciers.

Small pieces of rock can be transported by wind, glaciers and rivers.

Task 2


Abrasion of rocks by water, wind and ice, causes rocks to become smooth and rounded as they bounce into one another. Take a look at your pet rock

2.1 Is your pet rock round and smooth, or jagged and pointy?

2.2. Do you think that your pet rock has been moved by water, wind or ice?

Task 3.

thinkingwriting taskpair-work-icon

A thinking, discussion and writing task.

Dr Wilkinson has set up a model river, in small groups take a look at the model river, and the pieces of rocks being moved and transported.

3.1 – Observe where the water is moving fastest, what size rocks can be found there?

3.2 – Observe where the water is moving slowest, what size rocks can be found there?

3.3 – Which rock size is moved the furthest along the model river?

3.4  – How far do you think your pet rock could have been moved?  How far could your rock have moved if there was a really big river? Or a glacier?

Materials From The Earth – Part IV – Rock Hardness & Chemical Weathering

Lesson Objectives – By the end of this lesson you will:

  1. test rocks for hardness and chemical weathering.
  2. understand that different rocks have different hardnesses.
  3. understand that some rocks are erroded quickly by acid rain.


Rock Hardness

You may have already noticed from holding them in your hands, that different rocks have different hardnesses. Small bits of some of them come off in your hands, whilst others are much harder to rub off grains.

Geologists (the name for people who study rocks) are able to order rocks in terms of hardness by scratching the rocks with different substances, or by scratching the rocks against each other. The harder rock will leave a scratch on the softer rock.

Task 1 – Order your rocks for hardness

girl experimentgroup work

This is an experiment and group work task.  (Note: If yours is a decorative rock from home, which you don’t want to get damaged, use a school rock for this test instead).

1.1 On your table gently scratch your pet rocks against each other.

1.2 Decide as a group, which one cannot be scratched by any of the others, this is the hardest rock.

1.3 As a Group decide on the answer to these questions:

  • Which rock type (igneous, sedimentary or metamorphic) is the hardest rock?
  • Which rock type (igneous, sedimentary or metamorphic) is the softest rock?

Acid Rain

Rainwater is naturally slightly acidic, the natural acids in rain can chemically react with types of rocks to wear them away. This can cause statues made from these rocks to wear away or for caves to form underground where one rock type has been chemically worn away over millions of years.

wookey hole

Wookey Hole caves are formed where Limestone is chemically worn away underground.

Acid Rain wore away this statue on Bath Abbey

Acid Rain wore away this statue on Bath Abbey


Task 2

girl experimentpair-work-iconwriting task

This is an experimental task, a pair-work task, and a writing task.


2.1 Add small drops of hydrocholoric acid onto a class rock.

2.2 Write down the name of the rock (and what type it is), and what happens when you added the acid (you may want to use a hand lens to look carefully).

2.3 Repeat 2.1 & 2.2 for other rocks.

2.4 Make a list of rocks which react with acid. Do they have anything in common?

2.5 Test your pet rock? Write down what happens when acid is added to your pet rock?

2.6 Make sure you rinse your pet rock before you put it back in your bag.

2.7 Rain is only slightly acidic, not as much as the acid we just added.  Why did we use such strong acid in class?


Task 3. Statues

Me no Dum Dum

“Me no dum dum”

thinkingwriting task

This is a thinking and writing task

You have been approached by a local sculptor to recommend a type of rock for his new statue, his statue is going to be outdoors, and needs to last a long time, but he doesn’t have much time to carve it.

3.1 What characteristics does the stone for his statue need to have?  Should the rock be hard or soft, should it react with acid or not react with acid?  Does it have to look nice?

3.2 Which type of rock would your recommend to the sculptor for his statue?


 Task 4 – Extension Task – Actual rocks used for statues.


Investigate on the internet, the types of rocks that are commonly used for statues and why these rocks are used.

Materials from the Earth – Part III – Rock Permeability

Lesson Objectives – By the end of this lesson you will be able to: 

  1. test rocks for their permeability.
  2. explain why the shape of rock grains make them permeable.
  3. predict whether your pet rock is permeable, and test that prediction.

Permeability  (per·me·a·bil·i·ty)

Permeability means how easily water can pass through something.  Some rocks are permeable to water.  But some rocks are not. Today we are going to investigate which ones are able to absorb the most water, and which can hold no water at all.
Task 1. Testing Rock Permeability 
girl experimentgroup work
This is an experimental task and a teamwork task.
For each rock that you test, you will need to do the following:
  1. Weigh the dry rocks on the balance. Note down the dry weight (A).
  2. Place the rocks in water for at least 10 minutes, to give them plenty of time to soak up any water.**
  3. Take the rocks out of the water and dry them on a paper towel to remove any surface water.
  4. Weigh the rocks again. Note down the weight after soaking (B).
  5. Calculate the mass of water absorbed (C), and the amount of water absorbed per gram.

Note down all your results in a table like this, in your books.


** – While you are soaking your rocks, you can check you have completed tasks from Lesson II to the best of your ability.

Task 2. Explaining why some rocks are permeable.

thinkingwriting task pair-work-icon

This is a thinking , discussion and writing task.

Take a close look at the rocks which were most permeable, what shape are the grains in these rocks?  Look at the grains of those rocks that are not impermeable (not permeable), what shape are those grains.

Dr Wilkinson is going to show you a demonstration that may help explain why some rocks are permeable, and others are not.

Watch the demonstration, then write down why some rocks are permeable and others are not.  Which types of rock are mostly permeable?

Task 3. Predict whether your pet rock is permeable or not, and test your prediction.

thinkinggirl experiment

This is a thinking and experimenting task.  From what you now know about rock permeability, examine your pet rock from home and predict whether it will be permeable or not.

  1. Write down your prediction.
  2. Using the same instructions as Task 1 – Test whether your rock is permeable.

Was your rock permeable? Was your prediction correct? If not, why not?

Materials From The Earth – Part II – Types of Rock

Lesson Objectives

  1. Describe the way different rock types are formed.
  2. Recall that rocks are made of grains.
  3. Describe the different appearance of grains in different rock types.


Did you know that there are three different types of rock?  The three different types of rock are called igneoussedimentary, and metamorphic. These classifications depend on how the rock was formed.

Igneous Rocks.

An igneous rock

Igneous rocks are formed when hot, melted rock underground (called magma) comes to the surface of the earth and cools down into a rock. This might happen just under the surface of the earth, or it might happen on top of the surface, following a volcanic eruption! When the magma cools down, crystals are formed in the rock, and this is how we identify igneous rocks.

This video explains how igneous rocks are formed.


Sedimentary Rocks

A sedimentary rock

Sedimentary rocks are formed when grains of rock (sediment) are washed into the sea.  The weight of more and more sediment landing on top causes the sediment grains at the bottom to stick together. This is a process that can take many millions of years to happen. The grains in sedimentary rock can be different sizes, but are often rounded. This is how we identify sedimentary rocks.

This video explains how sedimentary rocks are formed.


Metamorphic Rocks

A metamorphic Rock

Metamorphic rocks are rocks that have been changed from one type of rock (either sedimentary or igneous) into a new type of rock. It’s pretty difficult to change rocks, it requires either huge pressures or very hot conditions. This happens deep underground, where the pressure of the rocks above and the heat from the centre of the earth change rocks from one type to another.  The pressure often makes the rock grains all line up, so often (but not always), metamorphic rocks have layers.

This video explains how metamorphic rocks are formed. 


Task 1

writing task


This is a writing task. In your book, draw a table comparing the different types of rocks (Igneous, Sedimentary & Metamorphic – these links are to BBC Bitesize and may also help).  Your table should include the following information:

  • how the rocks are formed
  • what visual characteristics do these rocks have
  • some common forms of these rock types (Extension)


Task 2

writing taskdrawing


This is a writing and drawing task.  For your homework, you brought in a pet rock to study during this unit.  Do the following things with your rock:

  1. Take a picture of your rock with your iPad.
  2. Draw a pencil sketch of your rock on one half of a piece of paper.
  3. Use a hand-lens to examine your rock closely – examine the types of grains that make up your rock
  4. Draw a close up sketch of the grains of your rock on the other half of the paper.
  5. Using what we have learned today about the different grains in different types of rock, write down whether you think your rock is an igneous, sedimentary or metamorphic rock.  Why do you think that?