Many science experiments exist that use some kind of liquid. While the children might not be able to drink the experiment, they will still have fun watching and participating. Pour some carbonated water into a clear glass and drop three to four raisins in there. Explain to the children how the air bubbles move the raisins.
Pour some milk into a glass and add a few drops of vinegar. The preschoolers will be able to watch the milk curdle. Pick up some of the curds to show the children. This would be a good tie-in if you're talking about Little Miss Muffet. However, tell them they should not eat curdled milk because it will make them sick.
Show preschoolers what happens when you add a few drops of food coloring to a glass of water. Give them each a glass of water and help them apply the food coloring. They'll love the colors they can make.
You also can show them what happens when you combine the same amounts of water and cornstarch together. The result is a thick goo.
Fill another glass with water and add an ice cube. Have the children place a string on the ice cube. Next, sprinkle some salt on top of the ice cube and have the children place a string on top of it again. This time, the string will stick. Explain to them how salt melts ice.
You can let your preschool students write their names or draw a picture on white printing paper with lemon juice. When their creations have dried, there will be no sign of them on the paper. Hold the papers up to a light bulb. The lemon juice will turn brown and the students will be able to see what they wrote or drew.
These experiments take more time than the others, but the children will be able to learn a lot by watching them as time goes on.
Get four or five clear jars and different leftover foods to teach your students about mold. Some good foods would be grapes, broccoli or cheese. Do not use any meat because it will stink after a couple of days. Dip each leftover into some water, then place the food into the jars. Seal the jars with tape and check them every day. First, the children will see the fuzzy mold appear, then the foods will begin to rot. Ask the students about the differences between the foods when that happens.
The students also can plant some vegetable seeds. Experiment with different amounts of water and sunlight and see which plants are growing better.