Oobleck --- a cornstarch-water mix named after green oozing stuff in a Dr. Seuss book --- is one way to demonstrate the properties of liquids and solids to kids ages 4 to 8. Mix 2 parts cornstarch to 1 part water; add a few drops of food coloring, if desired. Let kids manipulate it and play with it. They'll soon see that the mixture pours like liquid but feels solid when squeezed. What makes the difference? Pressure. Under pressure, the starch mixture behaves as a solid. When pressure is released, it behaves as a liquid.
Kids ages 8 to 10 (or younger) can write their own messages in invisible ink. To conduct the experiment, give kids clean white paper, lemon juice and paintbrushes or cotton swabs to write with. Have them write "secret" messages on the paper, then wait until the juice dries. Adults then help kids hold their papers next to an incandescent light bulb. The heat from the bulb burns the juice, darkening it so it can be read.
No two substances have produced more science class foam than vinegar and baking soda. Here's an outdoor experiment that launches a small (1/2-cup or so) plastic food container with a tight-fitting lid. Pack 2 tablespoons of baking soda tightly into the bottom of the container. Quickly pour in two teaspoons of vinegar and snap on the lid. Put it on the ground, lid-side down, and stand back. The reaction should launch the container several feet into the air. Repeat the experiment several times using different amounts of vinegar and soda and record the results.
Cut a large potato in half to create two cells for your battery. You'll also need two galvanized nails, two pennies and several pieces of wire, 3 to 4 inches long. Wrap one end of a piece of wire around a penny. Repeat with another wire and another penny. Use a sharp knife to cut a slot into each potato half for a penny. Push one nail into each potato half, making sure not to touch the pennies. Connect the end of a wire from one penny to the nail in the other half of the potato. Wrap a wire tightly around the remaining nail. One potato half should have a penny with a free wire. The other should have a nail with a free wire. Touch the base of a small LED bulb to the wires to create a glow of electricity.