Why does water boil?
The short answer first, then the real reason — and a version for younger kids. Guess before you scroll to make it stick.
✅ It gets hot enough for bubbles of steam to form inside
But why?
Water molecules are always jiggling — heat just makes them jiggle harder. Boiling is the moment the jiggle wins a contest against the air. Every molecule in the liquid feels two forces: the atmosphere pressing down on the surface, and its own urge to escape as vapor. Heat raises that vapor pressure. At exactly 100°C at sea level, the push from inside matches the push from outside — and suddenly bubbles of pure steam can be born inside the liquid itself, not just at the surface. They grow, rise, and burst: the rolling boil. The first tiny bubbles you see while warming up are a different show — dissolved air sneaking out. Real boiling bubbles are pure steam. Pressure writes the rules. On Mount Everest the air presses half as hard, so bubbles win at only ~70°C — water boils cooler and cooks noodles badly. In a pressure cooker, trapped steam raises the pressure, the boiling point climbs to ~120°C, and dinner cooks faster. And the water never gets hotter than its boiling point — a rolling boil isn’t hotter than a simmer, just more violent. Microwave warning: a perfectly smooth cup can superheat water past 100°C with no bubbles — which then flash-boil violently when disturbed. That’s why you tap the cup before heating.
🧑💼 For grown-ups: the deep dive
Boiling begins when vapor pressure equals ambient pressure (Clausius–Clapeyron relation). Bubbles need nucleation sites — scratches and pot edges boil first; perfectly smooth vessels superheat. Above the Leidenfrost point, a vapor film forms and behavior changes entirely.
🧒 For younger kids: At 100°C, water turns to steam INSIDE the pot — bubbles of steam rise up and roll!
🤯 Wow fact: At the top of Mount Everest, water boils at only 70°C — thin air means weak bubbles (and sad, undercooked noodles).
Good guesses kids make
“Because the pot shakes it” — You can boil water in a calm microwave too — heat, not motion, makes boiling.
“Because air gets mixed in” — The first tiny bubbles ARE dissolved air leaving — but true boiling bubbles are pure steam.
“Because water disappears into the pot” — Water doesn’t vanish into metal — it turns to gas and leaves as steam.
📚 Sources
More science at home why questions
- Why does your tongue stick to ice in winter?
- Why do you get a tiny shock from a doorknob?
- Why can you smell cookies from another room?
- Why is snow white when water is clear?
- Why is fire hot?