# How

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## What are stars made of? Astronomy, physics

By | 2017-10-14T16:41:05+00:00 August 19th, 2017|How, Physics|

The sun from space The earliest stars probably formed only a few hundred thousand years after the Big Bang that started the universe - maybe about 13.5 billion years ago. Stars are the oldest big objects in the Universe. Stars formed big clumps of hydrogen and a few helium atoms, the simplest kinds of atoms, and [...]

## How old is the Earth? – Astronomy

By | 2017-10-17T13:03:27+00:00 August 18th, 2017|How, Physics|

Earth from space Like Mercury and Venus and the other planets that go around the Sun, the Earth probably formed a little more than four and a half billion years ago. (That's about 4,600,000,000 years ago.) Like the other planets, Earth is made of molecules that were floating around in space. They made a nebula cloud of dust that was the leftovers when our [...]

By | 2017-10-14T16:39:03+00:00 July 29th, 2017|How, Math|

A number line is infinite in both directions. Adding numbers is like walking along a path, the number line. To add two small numbers, like 7 + 4, you can just start from the 7 and walk forward 4 spaces, and then see where you are on the number line, and there you are [...]

## Sundial project – How to make a sundial

By | 2017-10-14T16:39:16+00:00 July 21st, 2017|How, India, Science|

Making a sundial in your yard You'll need to do this project on a sunny day. Get a wooden stick, about a meter long, and put it in a place that will be sunny all day, like a south-facing windowsill or the middle of your yard. At the midpoint of the wooden stick, [...]

## How did ancient Greek people write numbers?

By | 2017-10-14T16:39:42+00:00 July 18th, 2017|Greeks, How, Math|

How the alphabet evolved People in ancient Greece had a variety of different ways of writing down numbers, but none of them was very efficient. (And of course different Greek city-states used somewhat different systems, too). Some Greeks used a system based on writing the first letter of the word for that number. Like in Greek [...]

## How to prove the Pythagorean Theorem

By | 2017-10-14T16:39:50+00:00 July 18th, 2017|Greeks, How, Science|

Diagram for proving the Pythagorean Theorem You can prove the Pythagorean Theorem for yourself, using the proof described on the Pythagorean Theorem page. Just get a square piece of paper, and draw the lines on it that you see in the third drawing (the drawing of a square). Draw the diagonal lines as in [...]

## How to build a shaduf – Ancient Egypt projects

By | 2017-10-14T16:40:13+00:00 June 10th, 2017|Crafts, Egypt, How|

Egyptian shaduf This is a project you should do outside on a nice warm day, because you are going to get wet! A shaduf is a machine to move water from a lower place to a higher place. You will need two large basins (or a creek and a basin), a step or [...]

## What is plastic? Organic chemistry

By | 2017-10-14T16:41:33+00:00 June 2nd, 2017|History, How|

Plastic water bottles There are thousands of different kinds of plastic, from shopping bags to Fisher-Price toys to photographic film, CD's, and plumbing pipes. Some plastics, like chewing gum or rubber, grow as part of plants, but people make most of the different kinds of plastic. We call them "plastics" because they are [...]

## How flowers reproduce – Plant Biology

By | 2017-10-14T16:38:02+00:00 May 30th, 2017|Biology, How|

Black-eyed Susan (the black in the middle is the seeds) The earliest flowers probably developed about 130 million years ago, in the Cretaceous period, as a way to attract newly evolved flying insects - mainly bees - and get them to help spread the plant's pollen far away from where the plant was [...]

## Evolution of Cells – Biology

By | 2017-10-14T16:38:21+00:00 May 29th, 2017|Biology, How|

A mutated snake with two heads With all these new molecules like lipids, amino acids, RNA, and protein floating around in the oceans by around four billion years ago, there were a lot of new possibilities for what could bump into what. Sometimes a group of lipid molecules happened to stick together and [...]