Place a peeled hard-boiled egg resting in the mouth of a glass jar or bottle. The egg must sit in the opening without falling inside. Have the children observe that the egg is just sitting there. Remove the egg and place a votive candle in the bottom of the jar. Light the candle and place the egg back on top. As the candle burns, it uses up the air in the jar reducing the air pressure. Since the pressure of the air outside the jar is greater, it pushes the egg inside.
Place about an inch of hot water inside the bottom of a glass soda bottle. Place a deflated balloon over the bottle opening and let it hang down inside the bottle. As the kids watch, put this bottle into a bowl of cold water and together observe how the balloon inflates inside the bottle. The scientific reason is that hot water heats the air inside the bottle and when this bottle is placed in cold water the air becomes cool. This creates a vacuum that causes the balloon to inflate.
Take a clear water glass and put 1 to 2 inches on honey in the bottom. Next place 1 to 2 inches of water and finish with 1 to 2 inches of cooking oil. The three liquids will form separate layers to make a water sandwich. Have the kids take a spoon and stir the liquids and watch how they make the layered sandwich again. The reason is that each liquid has a different density. The honey being the most dense sinks to the bottom whereas the water is next and the oil is the least dense and always floats to the top.
Pour a small amount of water and 1/2 cup vinegar in a glass wine bottle. Take the cork from this bottle and wet it with water. To do this experiment, put a teaspoon of baking soda into a wad of tissue paper. As the kids watch from a distance, drop in the baking soda wad and place the cork back into the bottle top. Stand back as the cork pops off the top. The vinegar and baking soda create a chemical reaction making carbon dioxide. As this gas fills the bottle, it forces the cork to fly off. Use caution with this experiment so children will not get hurt.