system
Given the following post, respond as if you're commenting on Reddit:
human
Why are clouds all fluffy on top but flat on the bottom?
gpt
A very cool way of looking at clouds is picturing them as the effect when warm moist air and cold air mix. Add in the fact that warm air rises because it's less dense, and warm sea-level-pressure air can hold more dissolved invisible moisture than cold lower-pressure air, and you have your ingredients to a flat-bottomed cloud.

First, what makes clouds visible? In a very easily repeatable experiment which you can see in just about any nature documentary when big mammals are doing stuff in a winter somewhere, their breath is very visible and forms a little cloud until it cools down and dissipates. 

What happened there is "fog", and its cause was warm moist air in the animal's breath holds more water than cold air, and when it cools down it has to surrender that moisture. So a region of warm moist air entered a cold zone and fell below the "dew point", the point when the moistness it was holding condensed out and formed tiny visible droplets. Lowered air pressure can also helps this condensation effect, which is why you'll see a little tiny cloud form on its wingtips when a jet fighter go fast enough because the air pressure there is really low.

So now let's look at clouds. 

Air generally gets cooler as you climb into the sky, and its pressure reduces. Sometimes it's not very disturbed and forms a clean gradient of temperature and pressure in the sky, both going down uniformly as you climb. On certain summery days when conditions are right, the level of the "dew point" is at a very flat, even height in the sky because the combination of temperature and reduced pressure is at a not-very-mixed-up consistent attitude.

So the sun shines, and warms stuff on the surface. If it's humid, warm moist air starts rising and rising... and breaks THROUGH that consistent dew point layer. 

The result? It condenses to cloud just above that specific "dew point" layer. And you get a cloud with a flat bottom.

To extend this, if the updraft is strong enough, moist air keeps shooting up and feeding the cloud's growth until you get highly energetic cloud systems with all sorts of cool stuff like rain, hail, lightning, thunder... and tornadoes (except they can sometimes be not so cool).