If you look at big fluffy clouds in the sky, you may notice that they’re only fluffy on the top, while the bottoms are pretty flat. That’s because the cloud isn’t just floating through the sky. It’s actually riding on top of a heavier layer of air!
Often we look at air (or as much of it as we can because it’s invisible) and we think that it’s the same all through the sky. Air actually changes a lot. There are heavier parts and lighter parts. Oxygen gas and nitrogen gas, two of the many gases that make up air, are actually heavier than the water vapor in clouds! It may seem weird to think about, but water is actually made up of lighter stuff than the gas in the air! It floats just like whipped cream does on hot chocolate. Only, imagine that the hot chocolate is invisible. You’d get something that looks a lot like a cloud: fluffy on top, flat on the bottom.
When water is a liquid though, the pieces of water get packed together very tightly and, together, they become heavier than the gases in the air. That’s when we get rain!