If you look at it from below, the clouds seem to be very light and just floating in the sky. Even though one large cloud can contain very large amounts of water.
Some large cumulus clouds can have a water mass of around hundreds of thousands of kilograms. The water is spread in small droplets over a large area, so we don't see it like an ordinary collection of water.
So clouds are not light objects that just hang there. There is a lot of water in it, but it is very small and scattered in the air.
