The Physics of Energy
 All About Vapor Pressure Why does boiling a liquid create vapor that can spin a turbine? Actually, all liquids have a certain "vapor pressure" at certain temperatures. For instance, a cup of water at room temperature has vapor pressure even though you don't see steam coming out of it. So what causes vapor pressure? Let's go back to the cup of water example. Look at the surface of the water. Now imagine zooming in really close. If you could see the individual molecules, you would notice that they were all moving around slightly, constantly jostling and shoving each other but staying in the same overall position. The movement is caused by the energy associated with the given temperature. At room temperature, the molecules will all have a certain amount of energy and will therefore be moving around at a certain speed, whereas at a colder temperature, the molecules will have less energy and will not be moving around as much. At absolute 0 (0 Kelvin, or -273 degrees Celsius), the lowest possible temperature, technically the movement of the molecules would be at a minimum (although there would still be movement). Now imagine that one of the molecules of water near the surface is pushed a little harder from below. The molecule gains just enough energy to "break free" of the other molecules and is launched into the air. The free molecule will most likely escape into the open air if there is no lid on the container; if there is a lid, however, then the molecule will bounce off of the different surfaces within the system and eventually return to the surface of the liquid where it "reattaches" itself to the other molecules.