Preschoolers won't yet understand that materials that take up the same space may have different weights. Sand and water are all you need to teach this concept. Ask children to predict what will happen to dry sand when water is added to it. Specifically, ask them to guess whether the sand will be heavier, lighter or the same weight. Mix half of your dry sand with water and give each child two cups. Ask children to fill one cup with dry sand and one with wet sand. Using scales, children will measure the cups. They may also draw conclusions based on how heavy the cups feel.
Using a microscope or magnifying glass will make preschoolers feel like real scientists, and it will teach them about all the components that can be found in a handful of sand. Pour some sand onto black paper so it stands out and let children take turns looking at it through the magnifying glass. Explain that sand can be made from broken rocks or from living organisms such as animal bones or shells. Use an eye dropper to drip vinegar onto the sand. If any of the bits of sand bubble, explain that these bits came from a formerly living thing.
With its thousands of tiny grains, a bowl full of sand can take a long time to transfer to a different bowl if you're using the wrong tools. Challenge preschoolers to solve this problem by giving them a variety of kitchen tools to use. Give each child a bowl of sand and one empty bowl and ask her to transfer the sand to the second bowl without using her hands or moving the bowls. As children try out various sizes of spoons and forks, they'll learn that the tool with the largest surface area moves the most sand at one time.
Even a task as simple as drying out sand forces preschoolers to think critically and creatively. Fill several bowls with damp sand. Gather children together and ask them to make suggestions of how to make the sand dry out. If children can't come up with any ideas, you can suggest some solutions such as putting a bowl outside in the sun, patting the sand with paper towels or pointing a hair dryer at it. Once children have guessed what the fastest way will be to dry out each bowl, test out your theories.