Why does water boil at 100°C?

Water boils at 100°C because at that temperature the vapor pressure of water equals atmospheric pressure at sea level. When you heat water, the molecules gain energy and start moving faster. At some point, those molecules gain enough energy to escape the surface and turn into vapor, but as long as the vapor pressure is less than the air pressure around it, the vapor bubbles collapse. When the temperature hits 100°C, the internal pressure finally manages to overcome atmospheric pressure and that's when you actually see bubbles rising up.

Now, that's specific to sea level. If you live at high altitude, water boils at a much lower temperature because atmospheric pressure is lower there. Like, in La Paz in Bolivia it's probably around 87°C or so. And if you put water in a pressure cooker, you increase the pressure on top of it, so water only boils at a higher temperature. That's why a pressure cooker cooks faster - the temperature can reach 121°C or thereabouts, depending on how much pressure you apply.

Have you ever stopped to think about what's happening inside the water when it heats up? Water boils at 100°C at sea level because that's the temperature at which the vapor pressure of water equals the atmospheric pressure pushing down on the surface of the liquid. Like, while the water is cold, the molecules are kind of lazy, vibrating slowly. As you heat it up, they gain energy and start moving faster, escaping from the liquid to become vapor. But then the atmospheric air pressure is there preventing that from happening on a large scale. When the temperature rises enough, the molecules gain so much energy that they overcome this "barrier" and vapor bubbles start forming inside the liquid itself - that's when you see that classic boiling.

The cool thing is that 100°C is specific to sea level. If you go up to a really high altitude, like a mountain, atmospheric pressure drops, and water boils at a lower temperature - maybe around 85°C or 90°C depending on how high you are. The opposite also works: if you put water in a pressure cooker, you increase the pressure, so water only boils at a much higher point, like 110°C or more. That's why cooking at altitude is more complicated, because the food doesn't spend as much time in contact with actually hot water.