What you Need
What to do
Put a very little water in the bottle.
Light a match, blow it out, and put it into the bottle while it is still smoking.
Put the lid on.
Squeeze the bottle as hard as you can.
Swirl the water around at the bottom.
Let go of the bottle.
Do you see anything strange?
What may happen
When you release the pressure the bottle goes misty and when you release it again it should go clear. You have made a cloud in a bottle.
If you try the same thing without the smoke it won't work nearly as well.
Why does it happen?
When you compress a gas it gets hotter (see the fire piston) and when you let it expand again it will get colder.
When a gas gets hotter more water can evaporate and turn into the invisible gas, water vapour. When you release the pressure and let the gas expand rapidly it will get colder again. This means it can't hold as much water vapour and it will try to condense back into liquid water.
It is very hard for the water vapour to condense in clean air, so it may condense on the walls, but if there are some smoke particles in the air, there are lots of places for water to form little drops - so you get lots of little drops of water suspended in the air - a cloud.
Does this have anything to do with real clouds?
The process is very similar to how clouds from on a sunny day. Warm moist air rises, and as it rises there is less air above it so the pressure reduces.
When the pressure reduces the moist air expands, getting colder. If there is some dust or pollution in the atmosphere the water will condense creating a cloud. If the droplets of water get big enough they become too heavy and will fall as rain.