Grown-up notes

Each line is marked Fact well established, Model a deliberate simplification, or Unsure not yet known.

Play another way

Another connection: Why is there day and night?

The idea

• FactWater is made of molecules, each one two hydrogen atoms joined to an oxygen atom (H₂O). On screen they are called "bits".
• FactIce, water and steam are the same molecules. What changes is how much they move and how they are arranged.
• FactIn ice the molecules are held in an open, six-sided framework and jiggle on the spot. In liquid water they stay close but slide past one another. In steam they are far apart and fast.
• FactThat open framework makes ice about 8% less dense than water (0.917 against 1.000 g/cm³ at 0 °C), which is why ice floats. The toy draws the ice pattern with gaps for this reason.
• FactAt sea level ice melts at 0 °C and water boils at 100 °C. Water also turns to gas well below boiling. That is evaporation, and it is how a puddle dries.
• FactSteam, meaning water as a gas, is invisible. The white cloud above a kettle is tiny drops of liquid water that formed when the steam met cooler air. Look for the clear gap right at the spout of a real kettle.
• FactThe molecules never stop moving, even in ice. Colder means slower, not still.
• FactFloating freshwater ice melting in fresh water leaves the level essentially unchanged: it displaced its own weight of water. The opening treats the drink like fresh water. In a denser drink, such as sugary cordial, a small level change is possible.

What the pictures simplify

• ModelSize and number. One drop of water holds more than a thousand billion billion molecules (about 1.7 × 10²¹). The toy draws 36.
• ModelColour and shape. Molecules have no colour. The blob with two bumps is roughly the outline of a water molecule, drawn as a cartoon.
• ModelSpeed. Real water molecules in air move at hundreds of metres a second. The toy slows them enormously.
• ModelFlat. Real ice is three-dimensional and each molecule holds on to four neighbours. The flat honeycomb shows three.
• ModelSpacing in steam. Steam at 100 °C takes up about 1,600 times the room of the water it came from. The toy cannot show a gap that large.
• ModelOnly water. A real pot also has air in it. The toy's lid keeps every bit in so she can see that none are lost.
• ModelSteered. The bits are nudged to behave like water. The toy is not a calculation from first principles.
• ModelThe puddle. In shade a puddle dries more slowly; it does not stop. Real rain comes from clouds that formed somewhere else, not the same bits dropping straight back.
• ModelThe kettle card. Ordinary room air holds some water as gas too, so the card shows a couple of bits there.

Questions she may ask

• Can we see a bit? FactNot with eyes or an ordinary microscope: a bit is far smaller than light can show. Special instruments can make images of individual molecules by scanning with an extremely fine tip; that is different from looking through an ordinary microscope.
• What are bits made of? FactEven smaller pieces called atoms. A water bit is three atoms stuck together.
• Is everything made of bits? FactEverything you can touch is: air, juice, the table, you. Different stuff has different bits. Light is not made of them. UnsureOut in space there is about five times more "dark matter" than ordinary stuff, and nobody knows yet what it is.
• Why does ice float? FactThe bits in ice hold each other in a pattern with gaps, so ice takes up more room than the water it came from and is lighter for its size.
• Where does rain come from? ModelWater bits in the air gather into tiny drops, and lots of tiny drops make a cloud. When the drops get big enough they fall.
• Do the bits ever stop? FactNo. Colder is slower, but they never sit completely still.

Words this guide teaches

Bits: the tiny pieces water is made of. Steam: water when its bits have flown apart; you cannot see it. Model: a pretend picture that helps us think about something we cannot see.

Go further

A Drop of Water: A Book of Science and Wonder by Walter Wick (Scholastic, 1997). Close-up photographs of water doing surprising things. The author suggests ages 7 and up, so it is one to look through together. Try your library first. About the book.

Sources

• American Chemical Society, "Condensation Station": drops on a cold cup come from water in the air.
• LibreTexts, "Structure of Ice": the open six-sided framework and the densities of ice and water.
• US Geological Survey, "Evaporation and the Water Cycle": evaporation at ordinary temperatures.
• Scientific American, "How does bathwater give off steam?": visible "steam" is liquid droplets.
• Gross and others, Science, 2009: the first picture showing the structure of a single molecule.
• NASA, "Dark Matter": most matter in the universe is of an unknown kind.

Melting and boiling points, the number of molecules in a drop, molecular speeds, the volume of steam and the floating-ice result are standard textbook values, worked out or recalled rather than taken from a linked page. Checked November 2026.