The "ear guitar" activity is an opportunity to teach preschoolers how vibration generates sound. You'll need a nail, two plastic cups, two paper clips, string and a pair of scissors. Use the nail to poke a hole in the bottom of each cup. Cut the string into a 15-foot length, and thread the ends into the cups through the holes. Tie the ends of the string to paper clips so they stay in the cups. Now two children can each hold a cup to one ear. Have one of them pluck the string: each will be able to hear it in the cup. Try loosening and tightening the slack in the string and notice how that changes the sound.
This activity helps children understand the roles density and surface area play in determining whether an object floats or sinks in water. Gather a wood block, a plastic cap from a bottle, two pieces of aluminum foil and a chunk of clay. Fill a tub or sink with water. Have a preschooler hold the block in one hand and the cap in the other, and ask which feels heavier, and whether either will float. Now try them both. With the aluminum foil, try the experiment with one piece shaped into a tight ball and the other folded into a boat. Do the same with the clay: once as a lump and once in the shape of a boat. You'll get kids thinking about why the same material floats one way, but not the other.
Growing alfalfa sprouts can help preschoolers learn how plants use sunlight to grow. You'll need about a tablespoon of alfalfa seeds, a clear glass jar, a piece of cheesecloth, scissors, a rubber band and water. Put the seeds in the jar and cover them with water, then drape the cheesecloth over the top and secure it with the rubber band. Let the seeds soak overnight and drain them the next day. Keep them in a cupboard for four days, adding water to keep the seeds damp but not soaking. After four days, set them on a window where they'll get plenty of sunlight. They will turn green, and the kids can eat them.
Colored frosting and graham crackers give preschoolers a sweet way to learn how blending primary colors can produce every other color. Mix white frosting with food coloring to make a cup each of red, yellow and blue icing. Students can predict which colors each combination will make, then use craft sticks to spread the colored icing onto graham cracker segments.