Here we have a cup of coffee. You know that if you leave a cup of coffee, as in temperature, eventually it will cool down. That's because energy transferred from the cup of coffee to the air, okay? What actually happens on a microscopic level is that the atoms, for example, diatomic oxygen, hit the surface of the water, okay? When they hit the surface of the water, they bounce back with a higher kinetic energy, okay? So the oxygen gain kinetic energy while this lost kinetic energy, so it lost the heat.