ELI5: Why does water boil at 100 celsius degrees?

Imagine water is like a bunch of bouncy balls all stuck together in a room. They're wiggling and jiggling, but they mostly stay close. That's water at room temperature.

Now, you start heating the room. That's like putting the water on the stove.

• The bouncy balls (water molecules) start bouncing around faster and faster. They're getting more energy.

• This energy makes them want to break free from each other. They're trying to escape the group.

• But the air above the water is also pushing down on them. It's like a lid on the room, making it harder to escape. This pushing down is called air pressure.

So, why 100 degrees Celsius? It's because that's the specific amount of energy the water molecules need at normal air pressure to change from a liquid (bouncy balls sticking together) to a gas (steam, bouncy balls flying everywhere). If the air pressure is lower (like on a mountain), the water will boil at a lower temperature, because it's easier for the bouncy balls to escape. If the air pressure is higher, it will boil at a higher temperature.

Think of it like this: it's the temperature where enough bouncy balls are so excited, they can finally push open the door (overcome air pressure) and turn into steam!