Many earth science concepts can be introduced at the preschool age. The water cycle can be explained and demonstrated by building a closed system that shows evaporation, condensation and precipitation. Simply place some water in a clear plastic bottle, and let it sit in the classroom. Encourage children to observe it periodically and talk about what's happening.
Children this age often fantasize about exploring space as an astronaut, so it is a perfect time to talk about our planet, the sun, the moon, and other planets in the solar system. Teachers can remind children to take care of our earth through recycling and conserving energy.
Preschoolers have a natural interest in animals. Build onto this interest by studying different animals' habitats and characteristics. Get supplies that enable students to watch the life cycle of the caterpillar or tadpole firsthand. In "Science in the Preschool Classroom" PBS Teachers suggests having a class pet for observation and learning. Students will also enjoy planting seeds and watching them grow.
Biology also involves the body and preschool teachers can teach children how the body works and how to keep it healthy with good nutrition and exercise. A discussion of germs during cold and flu season can also help children learn about their immune system.
Physics seems like a difficult topic for preschoolers, but there a lot of common-sense principles, as well as simple experimentation, that children will enjoy. They can experiment with static electricity by rubbing a balloon on their head, or use magnets on different surfaces to determine how magnetism works. Let them play with small, safe mirrors to learn about reflection, and launch small marbles off a track to study projectiles. Provide many safe objects for them to drop off a high point to learn about gravity and its effect on different-sized objects.
Children any age enjoy watching chemical reactions, but it's best to keep them tame for this age group. Involve children in a baking project to show them how combining ingredients can make something new.
Science for Preschoolers suggests placing a tbsp. of baking soda in a balloon and an inch of vinegar in a bottle. Place the balloon over the mouth of the bottle, let the baking soda fall in and watch the resulting carbon dioxide blow up the balloon.
Preschoolers will also enjoy learning about phases of matter. Show children how water can be a liquid, solid, or gas by making ice and observing how water in a bowl evaporates.