Evaporation

Real World Example

Imagine trying to find the right key from a cluttered keyring to unlock your front door. Just as the right key needs to be selected and used to unlock the door, in evaporation, only the most energetic molecules at the surface of a liquid have enough energy to escape and become gas. The keyring represents the liquid, with each key as a molecule, and just like selecting the right key to leave the keyring, only the topmost, energetic molecules can break free from the liquid's surface to enter the air as vapor.