Magnets are clean and neat. They can be explained, but it's best to allow hands-on exploration for preschoolers to understand magnets.
Offer a variety of magnets and a collection of odds and ends. Things like paper clips, coins, plastic items, bottle caps, keys and other items found around the home or classroom.
Draw a racetrack on the interior lid of a shoebox. Place a metal car on the track and then show the preschooler how to make it move with a heavy magnet underneath.
Make your own iron filings by scraping a nail on a rough metal file. Place these in a sealed plastic sandwich bag that has been taped flat to a piece of card stock. Children can watch the iron filings move with a magnet under the paper.
Providing a place for students to examine seeds and plants is a useful science center. Ask the class to bring in seeds from home. They can bring seeds from their apples or other fruit, vegetable seeds or seeds from their yard. Help them sort and classify the different varieties of seeds. Provide magnifying glasses and empty egg cartons for sorting.
Plant alfalfa seeds in half an egg shell. This little project brings a little bit of art into science as the students draw faces on their eggheads. Keep the egg shell standing up by making a thin ring of cardboard for the egg shell to set inside.
Collect empty film containers or other small plastic jars. Poke holes in the lid and then fill with a cotton ball that has been soaked in vanilla, peppermint extract or perfume. Bits of onion, orange slices, cloves, cinnamon, lemon or oregano can be added to individual containers to make a smell station. The preschoolers can use their sense of scent to try to figure out each of the smells.
Shakers can also be made from small yogurt containers. Fill with items like rice, paper clips, marbles, toothpicks, salt, cotton balls, water or nothing at all. Seal the lid tight and let them figure out what the contents could be.
Set up the station so that the preschoolers can play a matching game by themselves or with a friend. Prepare pairs of identical scent containers and pairs of shakers. Let them place in similar groups.