Almost all the air around us is moist. That means that it contains water in the form of vapor. You can't see it because water vapor is a gas, but it's still water. Water can exist in three states; liquid (water), solid(ice) and gas(water vapor). Obviously, you can see and touch water and ice, but water vapor has no smell, you can't pick it up, and it's invisible. That doesn't mean that you can't feel it though.
Perhaps you can remember a hot and sticky day in summer, or a cold foggy day in winter, or even being in a hot shower full of steam? In each of those situations you will have felt water vapor all around you. If you stopped at really looked at that fog or steam you would have spotted millions and millions of tiny water droplets floating in the air. What you saw was the same process that makes clouds - millions of tiny water droplets condensing out of the air to form liquid water.
We've all seen fog and steam, but why does water condense out of air and become visible? Well, warm air can hold more water vapor than cool air, so if warm air starts to cool, it can no longer hold as much water vapour. The extra water vapour has to go somewhere, so it condenses out as water. So... cooling the air reduces it's ability to hold water vapour, and triggers the formation of water droplets. Remember that bit, 'cos it's very important.
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