Molecules are bouncing around in liquid water at a certain speed. This speed can be measured as the temperature of the water. Sometimes a molecule that is not really going fast enough to be vapor bounces near the surface and flies into the atmosphere, becoming vapor anyway. That’s evaporation. If you heat the water until all the molecules are going fast enough to become vapor, that’s boiling. The bubbles in boiling water are small pockets of water molecules that had absorbed enough energy to vaporize, lowering their density and allowing the molecules to float.
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