"How to boil water"

There's a saying in physics that just because something is simple, it doesn't mean it's easy. For example, "How does gravity work?" is a simple question, but even the likes of Newton and Einstein couldn't fully crack it.

Another deceptive question was sent to me the other day: when water is boiling, what's exactly coming out of the bubbles on the surface? I mean

Let's start with convection. When you heat a fluid on one side but keep it cool on the other, it will naturally start to mix with itself. That's because random blobs of fluid at the bottom will heat up just a little bit more than average. Heating up, they expand and become buoyant, rising to the surface. Once there they cool off and slink back down.

We see this same process all over nature. Pot of boiling water? Convection cells. Surface of the sun? Convection cells. Weather on the Earth? Convection cells. You get the idea.

Given enough energy, the water molecules start to get a little frisky, going from liquid to gas. This doesn't happen at the same time all across the pot of water. Again, randomly little blobs here and there will go gassy, expanding and rising to the surface when they do. But once there instead of sliding back down, the gas is free to escape, sending little packets of water out into the world in the form of water vapor.

So that's what's in a bubble of boiling water: water!