Obtain a clean, empty long-necked bottle with a mouth slightly narrower than an egg. Keep a cool, hard-boiled egg nearby with its shell removed. Light three matches (or allow the child to do so, if old enough) and quickly drop each one into the bottle in succession. When all three matches are lit and inside, place the egg atop of the mouth of the bottle. The flames will extinguish. If nothing happens after a few moments, try again with new matches. The egg will be sucked down into the bottle. This happens because the warm air from the matches expands inside the bottle, then cools when the flames are extinguished. The cool air contracts, creating an effect similar to a vacuum, sucking the egg into the bottle.
Allow the child to blow up a large balloon and tie it (or you can do this if the child is too young). Place a funnel into the opening of a similar balloon and add 1/4 cup or so of water. Blow up the balloon the rest of the way and tie it. Everyone participating in the experiment should wear protective goggles. Have the child hold the first balloon by the knot. Light a match and bring it near the balloon. Soon, the rubber will melt a bit and the balloon will pop. Now have the child hold the second balloon by the knot. Light another match and bring it near the water-filled portion of the balloon. The balloon will stay cool and will take a lot longer to pop, if it pops at all. Water takes a long time to heat to the point that the rubber outside it will melt. Air does not offer this thermal protection.
Place a dinner plate or disposable (non-paper) plate on the table. Pour a small amount of milk into it -- enough to cover the entire bottom area. Ask the child or children to squirt a drop of food coloring into the milk, then another drop of another color close to but not touching the first drop. Repeat with a third and even fourth color. Ask what will happen if a cotton swab is touched to the surface of the milk. Allow the child to gently poke the milk with the swab, but not stir the milk or colorant. Squirt a drop of dish soap onto the end of a fresh cotton swab. Have the child touch the soapy swab to the milk surface, holding the swab in place for 10 to15 seconds. The colors will swirl and mix seemingly on their own. The soap changes the surface tension of the milk and alters the fats and proteins within the milk, which "stirs" the molecules of the milk and food coloring together.
Cut a large lemon in half. Give both halves to the child and ask him or her to squeeze as much juice as he or she can into a clean glass, using both lemon halves. Pour an equal amount of water on top of the lemon juice. Allow the child to measure 1 teaspoon of baking soda and stir it into the lemon water mixture. Ask the child to taste the mixture. Stir in a very small amount of sugar if you think it needs to be sweeter. The lemon concoction is very similar to a natural soda, without any added artificial ingredients. The acid of the lemon juice mixed with the base -- baking soda -- creates a fizzy reaction, much like the carbonation found in a store-bought soda. Carbon dioxide is responsible for the fizz in both homemade lemon drinks and in store-bought sodas.