system
Given the following post, respond as if you're commenting on Reddit:
human
Why are the bottom of clouds flat?
gpt
Clouds are formed by rising air. The air can hold a certain amount of moisture based on its temperature, and the cooler it gets, the less water it can hold. 

As air rises, it cools. Eventually it gets to a point where it gets too cool for the amount of water already in the air, and the water vapour condenses out and becomes cloud.

For a large airmass, the height it reaches this temperature is uniform, hence the base of the cloud being flat.