Humidity

Real World Example

Imagine trying to fit all your groceries into a small tote bag. Just like you can only pack so many items before the bag becomes too full, the air can only hold a certain amount of water vapor before it reaches its limit. The tote bag is analogous to the air, and the groceries are like the water vapor; the more groceries (or water vapor) you add, the closer you get to the bag (or air) being full.