Here's why:

* Cumulus clouds are puffy, white clouds that look like cotton balls. They form when warm, moist air rises and cools. As the air cools, the water vapor condenses into tiny water droplets, forming the cloud.

* The flat bottom of a cumulus cloud is the point where the rising warm air meets the cooler air above. The warmer air is buoyant and continues to rise, leaving the cooler air below untouched. This creates a distinct boundary between the cloud and the clear sky beneath it.

Other types of clouds, like cirrus or stratus clouds, may appear flat, but they don't have the same distinct flat bottom as cumulus clouds.