It's the outside. And that's because those molecules found inside gain enough kinetic energy, they become gas molecules, and they travel up. The Boiling point is also the point at which vapor pressure created by the liquid is equal to or greater than the total pressure found above the system. So the atmospheric pressure. So, for example, suppose this is our system and this is our vapor pressure created by the system, and this is our vapor, our pressure, our atmospheric pressure. Now, if our atmospheric pressure is greater than our vapor pressure, pressure created by a liquid, the vapor pressure will not allow these molecules to escape.