system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does water boil when in a vacuum.
gpt
Water molecules are bouncing around with enough energy that they can move past each other.  This is why it is liquid.

Air molecules are bouncing around with enough energy that they can detach from each other.  This is a gas.

The air molecules bouncing around on top of the water molecules puts enough pressure on the water molecules that they cant escape the surface. 

In a vacuum this pressure does not exist, so the water molecules escape the liquid surface.  This is boiling.

If you want water to boil in air, then you need to add energy (heat) to the water to give the molecules enough bounce to push through the air.