We know that water can be a liquid, a gas, or a solid. Outside, water is always changing from liquid to gas and back again. This process is called the water cycle. You can see how the water cycle works.
The sun's heat causes water to evaporate from streams, lakes, rivers, and oceans. The water vapor rises. When it reaches cooler air, it condenses to form clouds. When the clouds are full of water, or saturated, they release some of the water as rain.
You will need:
The "mist" that forms on the plastic wrap will change into larger drops of water that will begin to drip. (You can speed up the dripping by carefully moving the bowl – don't splash! – into the shade.) When this happens, continue watching for a few minutes, then carefully peel back the plastic. Is the coffee mug still empty? Water from the "ocean" of water in the bowl evaporated. It condensed to form misty "clouds" on the plastic wrap. When the clouds became saturated it "rained" into the mug!
This experiment adapted from resources provided by the Monroe County Water Authority - http://www.mcwa.com/MyWater/KidsWaterFun.aspx#cycle