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It rains. There is a puddle by the door. The next day it has gone, and nobody drank it. So where does a puddle actually go?
Next time it rains, pick one puddle. A good one. Stand next to it and remember how big it is.
Go back later that day.
It is smaller. The edges have crept inwards.
Go back the next morning and quite often there is nothing there at all. Just dry ground, as if it never happened.
That is the usual answer. The water sank down into the ground.
Sometimes a little bit does. But it cannot be the whole story, and here is how you can tell.
After it rains, go and look at the flat lid of a bin. Or the roof of a car. Or the bottom of a plastic slide.
There are puddles on all of those. And a bin lid does not soak up water — that is its whole job. It keeps the rain out of the bin.
Come back tomorrow. Those puddles are gone too.
Water can leave a puddle without going downwards at all.
The water leaves from the top.
It does not leave all at once, and it does not leave in drops. It leaves in pieces so small that nobody can see one, not even under a magnifying glass.
Those pieces float away into the air. One, then another, then another, all day long.
The sun helps. Warm water lets go of its pieces faster than cold water, which is why a puddle in the sun goes quicker than one in the shade.
Wind helps too. It sweeps the pieces away and lets more leave.
The puddle is not being drunk and it is not sinking. It is floating off into the air, in pieces far too small to see.
Same rain, same day, four different spots. Take them one at a time and see if you can guess before you press.
There is a puddle sitting on the flat metal lid of a bin. In the morning it has gone. Where did that water go?
Take a cold bottle out of the fridge and put it on the table. Wait a few minutes.
The outside goes wet. Sometimes proper drips run down it.
The lid is shut. Nothing leaked. That water was already floating in the air of your kitchen, invisible, and the cold bottle made it show up again as drops you can see.
You have watched this before. Breathe on a cold window and it goes cloudy. Have a hot shower and the mirror disappears behind a fog.
Cold turns the invisible bits back into water you can touch.
High up it is very cold, so the bits that floated off your puddle do exactly what they did on the bottle, and a cloud is a huge crowd of those tiny drops.
When enough of them join up they are too heavy to float, and down they come as rain — making a new puddle by your door.
Puddle to invisible air to cloud to rain to puddle. The same water, going round and round, forever.
Every one of these is water moving. Some of it is leaving as invisible bits. Some of it is showing up again as drops you can see. Tap each one into a box.
Tap an item, then tap where it belongs
The cold bottle is soaking wet all down the outside. Its lid is screwed on tight and it is still just as full. So where did the water on the outside come from?
You’re previewing as a parent — nothing here is recorded.
That’s the last one in this channel for now. More are on the way.
Back to all episodesEvaporation: liquid water at any temperature loses fast-moving molecules from its surface into the air as water vapour, which is invisible. Puddles therefore shrink from the surface regardless of what they are sitting on (USGS Water Science School, 'Evaporation and the Water Cycle'; NOAA National Weather Service, JetStream). Rate of evaporation rises with higher temperature, greater wind, and lower humidity, and falls when the surrounding air is already near saturation — which is why a puddle vanishes in sun and wind but barely shrinks on a still, damp, overcast day (USGS Water Science School; NOAA JetStream, 'Moisture'). Condensation: when moist air is cooled, water vapour changes back into liquid water on the cooled surface. This is the mechanism behind droplets on a cold drink, a fogged bathroom mirror, and breath on a cold window (NOAA National Weather Service, JetStream, 'Condensation'; USGS Water Science School, 'Condensation and the Water Cycle'). Clouds are made of very large numbers of very small liquid droplets (and, when cold enough, ice crystals) formed when rising air cools and its water vapour condenses. The droplets are small enough that they remain suspended (NASA Earth Observatory; NOAA JetStream, 'Cloud Types'). Precipitation occurs when cloud droplets collide and coalesce into drops heavy enough to fall against the rising air. The water then returns to the surface, completing the cycle (USGS Water Science School, 'Precipitation and the Water Cycle'; NOAA JetStream). Water on Earth is continuously recycled through this cycle rather than created or destroyed — the same water evaporates, condenses and falls repeatedly (USGS Water Science School, 'The Water Cycle').