system
Given the following post, respond as if you're commenting on Reddit:
human
Why are clouds flat on the bottom? And when it rains do clouds get smaller?
gpt
Generally speaking (that is, the common/simplified model of cloud formation), because the "bottom" of clouds usually represents the condensation line where rising moist air has cooled enough to condense into cloud droplets. 

While it looks like a "hard" line, it's not like one side is moist and the other is bone dry. It's just on one side it's not quite cool enough to condense droplets out of air, on the other it is.