Escaping one at a time
Where Does a Puddle Go?
The fastest water molecules break free into the air — no boiling required.
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Step 1 of 5
Water molecules are always moving.
Even in cold water, molecules jiggle constantly. Some move slowly, some fast — temperature is just the average of all that motion.
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Step 2 of 5
The fastest ones at the surface escape.
A molecule at the top moving fast enough breaks free of its neighbours and leaves as vapour. This happens at any temperature.
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Step 3 of 5
Losing the fast ones leaves the rest cooler.
The escapees carry away above-average energy, so the water left behind is colder. That is why sweating and wet skin cool you down.
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Step 4 of 5
Heat, wind and area all speed it up.
Warmth gives more molecules escape speed. Wind clears away the escaped ones. Spreading the water thin exposes more surface.
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Step 5 of 5
Boiling is the same thing, everywhere at once.
Evaporation happens only at the surface. Boil it and molecules break free throughout the liquid, which is what the bubbles are.
Wet washing dries on a freezing day because dry cold air still accepts vapour — as long as it is dry.
The short version
Evaporation is the fastest surface molecules escaping into the air, which is why the water left behind gets cooler.
Try it yourself
Wet the back of one hand and blow on both. Only the wet one feels cold — that is evaporation removing heat.
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