system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 - Why are all the bottoms of clouds have a flatness about them while the tops are very non uniform?
gpt
Think about a bubble bath. All the foam is very irregular on top, but flat on the bottom. 

The same happens to clouds. When the air parcel reached a level where it can exist (defined by pressure, temperature and humidity), then it condensates and the clouds is formed. The interface separating the "here you can exist - here you can't" is the flat bottom. Same as the bubbles can't exist below the water, but exist above, and the interface is flat.