After introducing your little ones to the water cycle and some basic terminology (such as evaporation, condensation, precipitation and collection), try this activity. Give each child a cotton ball and tell them to pretend they're holding a cloud. Talk with them about the cotton ball in their hand and ask how it feels. Have them gently place their cotton ball in a shallow pie pan filled with water (that you can pretend is a river or ocean) and discuss that water evaporates into clouds just like the water is going up into their cotton ball. Children can then pick the cotton ball back up as you discuss how it feels different from the way it felt before it got wet. Explain to them that just as the water is too heavy for their cotton ball (water drips out of it), real clouds cannot hold all the water they absorb and that's why we experience rain.
This activity is useful after an explanation that water flows downhill. Use some of the blocks in your classroom to raise one end of a rectangular plastic storage bin (just stick some of the blocks under one end so that the bin is angled). Before you pour water into the bin, ask students to predict what will happen. Pour the water in and have your preschoolers observe how the water flows. Then ask them to figure out how to get the water to flow in the opposite direction (uphill). Have a reflective discussion with them. Ask if they could get the water to go uphill, do they think it would stay there since they've just observed that water goes downhill naturally. When you're done, have them illustrate this experiment on a piece of white paper with a blue crayon. Guide them in drawing what they've just observed.
This simple experiment gives preschoolers a chance to take note of precipitation. Freeze a small (about 8- by-11-inch) mirror before starting this experiment. Let the children feel the mirror to confirm that it is cold. (If this will take awhile with your group, rendering the mirror room temperature by the time you finish, you may want to freeze two mirrors; one for them to feel, one for the experiment). Heat a kettle filled with water. Talk with your preschoolers about the steam that comes out of the spout. Remove the kettle from the heat source and set it on a table, next to a piece of blue construction paper. Hold the mirror in the path of the steam and over the paper. Point out that the water on the mirror beads up and drips onto the blue paper, explaining to them that precipitation drips like this onto the ground outside and is used to nourish plants and other land animals.