Put out small bowls of materials such as flour, sugar, paint, sand and oil. Give each child a small bowl of water and ask him to choose a material to mix with his water. Each child must first make a prediction about what will happen when he combines it with water. Give children plastic spoons or let them use their fingers to mix the two materials together. Ask children to explain what actually happened. Repeat the experiment several times so each child gets to try several materials.
Teach children how water moves through a plant. Bring in one stalk of celery for each child. Give each child a cup of water and let her add a few drops of food coloring. Cut a bit off the bottom of each stalk. Children will put their celery in the water, then make a prediction of what will happen. After a day or two, cut the stalks in half so children can see how the dyed water moved up into the vegetable.
Collect empty jars, milk jugs or any other plastic containers of various sizes. Choose one jar to fill with water. Let each child look at the filled jar, then pour the water out. This will give them an idea of how much water the jar could hold. Next, ask children to choose another container that will hold more water than the jar. Let the children fill that container to the top with water, then pour it into the jar to see if they were right. Continue with other containers to test how much volume each one will hold in comparison to the jar.
Explain to preschoolers that cold water is heavier than warm water. Fill a large punch bowl or aquarium halfway with warm water. Add red food coloring until the water is saturated with color. Next, fill a pitcher with cool water and ice cubes. Stir the water until the ice melts, then add blue food coloring. Ask children to guess what will happen when you add the cold water to the warm, then slowly pour in the cold water. The blue water should sink and form a layer under the red water.