Where does the puddle go?
It rained in the morning. By evening the courtyard is dry. Nobody mopped it, and it did not run anywhere — the puddle was in a dip.
The water went into the air. At the surface of any puddle, some water is always slipping away into the air as an invisible gas — and the warmer and drier the day, the faster it happens.
The important part, and the bit that surprises most children:water does not have to boil to leave. A puddle at thirty degrees empties itself perfectly well without ever bubbling.
Try it today
Pour the same amount of water into two saucers. Put one in the sun and one in a cupboard. Check them at bedtime. Same water, same saucers — and one is empty.
The word for it: evaporation — water leaving as an invisible gas.
Why does the cold glass get wet outside?
Fill a steel glass with cold water on a summer afternoon. Within minutes the outside is wet, and there is a ring on the table. Children almost always guess it leaked.
It did not. The water on the outside was never inside the glass — it came out of the air in your kitchen.
Warm air carries a great deal of invisible water. When that air touches the cold steel it cools down, and cool air cannot hold as much. The water it can no longer carry appears as drops on the nearest cold surface — the outside of your glass.
Try it today
Wipe the outside of the glass completely dry, then watch it get wet again in a few minutes. Nothing was added. If your child still suspects a leak, fill a second glass with room-temperature water — that one stays dry, and the argument is over.
The word for it: condensation — the opposite of evaporation.
How does the monsoon get water into the sky?
Enough rain falls in a single monsoon to flood streets across the country. It has to get up there somehow first.
It is your saucer and your cold glass, at an enormous size. The sun heats the sea and water evaporates from it constantly. Winds carry that damp air over the land. When the air rises and cools, the vapour turns back into droplets — exactly what happened on your glass — and a cloud is nothing more than a great many of those droplets together.
When enough droplets join, they become too heavy to float, and fall. That is rain: water that left the sea, travelled as invisible gas, and changed its mind.
Try it today
Ask an adult to hold a plate of cold water above a pan of steaming water — well back from the steam. Drops gather on the underside of the plate and eventually fall. Evaporation, condensation and rain, in one saucepan.
An adult holds the plate. Steam burns badly, worse than boiling water. Your child watches.
The words for it: the water cycle — evaporation, condensation, rain, and round again.
Common questions
›Where does a puddle go when it dries up?
Into the air, as invisible water vapour. Water does not have to boil to leave — at any temperature some of the water at the surface escapes into the air, and on a warm dry day it happens fast enough to empty a puddle in an afternoon. It has not soaked away or disappeared. It is in the air around you, which is also why washing dries and why the floor dries after mopping.
›Why does a cold steel glass get wet on the outside?
Because the air around it is carrying invisible water, and cold makes that water turn back into drops. Warm air can hold a lot of water vapour; when it touches the cold surface of the glass it cools, cannot hold as much, and the extra appears as droplets on the outside. Nothing leaked through the steel. The water was in your kitchen air the whole time, and the cold glass simply made it visible.
›How does the monsoon get so much water into the sky?
The sun heats the sea, water evaporates from it endlessly, and winds carry that moist air over the land. When the air rises and cools — over hills, or as it climbs — the vapour turns back into droplets, exactly as it does on a cold glass, and those droplets are what clouds are made of. When enough of them join together they become too heavy to stay up, and it rains. The monsoon is the same two processes in your kitchen, at the size of a subcontinent.
›Why does a matka keep water cold without electricity?
Clay is slightly porous, so a little water seeps through to the outside surface and evaporates there. Evaporation takes heat away with it, and that heat comes from the water still inside. It is the same reason sweating cools you. One honest caveat worth telling your child: it works best in dry air. During the humid part of the monsoon a matka is noticeably less impressive, because the air is already too full of water to accept much more.
More water, in a bucket
Once evaporation makes sense, floating and sinking is the natural next argument to have with a bucket and ten things from the kitchen.
The float-or-sink kitchen lab →