Give preschoolers an introductory lesson to chemistry by cleaning pennies. Ask children how you might get dirty pennies clean. Try washing some in soapy water or wiping them with paper towels. Mix together one teaspoon of salt and 1/4 cup of white vinegar in a cup. Pour in some other dirty pennies and watch as they get clean. Rinse them in clean water, then compare them to the other pennies you cleaned. Children won't understand an explanation about why this worked, but it will introduce them to the concept of chemical reactions.
Teach children that there are all kinds of particles traveling in the air. Cut several pieces of wax paper to be about a foot square. Punch two holes in one side of each piece of paper and tie on pieces of string. Let children spread out a layer of petroleum jelly on each piece of paper. Go outside and hang the papers up in different locations. Hang them near buildings, parking lots and in wooded areas. Retrieve them the next day and examine them with magnifying glasses. Ask children if they see a difference in the particles depending on where the paper was located.
Show children how water moves through plants. Fill two plastic cups with water. Add several drops of food coloring to one cup and leave the other clear. Cut slits in two pieces of celery so the bottom halves are split in half. Ask children to guess what will happen, if anything, when you leave the celery in the cups overnight. In the morning, they'll see how the colored water moved up the celery. Explain that the same thing happened in the cup of clear water.
Most preschoolers enjoy building. Put out a variety of supplies such as empty cardboard tubes and boxes, toothpicks, wooden craft sticks, foil, paper, pipe cleaners, glue and tape. Challenge children to build a container that will keep an egg safe when it's dropped. Once each child has completed his creation, take everyone outside. One at a time, place a raw egg in each child's container and drop it from shoulder level. Once you've seen which containers kept the egg protected and which allowed it to break, ask children to notice any similarities among containers that were successful.