For any of these experiments, it's a good idea for an adult to supervise children, to always wear safety glasses, and if working with fire, to have a fire extinguisher handy.
Amaze Your Friends With a Fireproof Balloon
For this experiment, all you need is a balloon and a candle. Fill the balloon three-quarters full with water, and top it off with air by blowing the balloon up as far as it will go. Tie it off.
Light the candle, then slowly lower the balloon over it. Behold, the balloon won't pop!
This is due to water's incredible ability to absorb heat. The water in the balloon disperses the heat generated by the candle, and keeps the latex of the balloon from getting hot enough to break. But, when the water in the balloon can't absorb any more heat from the candle, the balloon will burst, and you'll probably get a little wet.
The Lava Lamp
From your kitchen, grab: a bottle of vegetable oil, food coloring, some salt, and either a large glass or a glass jar.
Fill the glass container 2/3 full of water and fill the remainder with vegetable oil. Add some food coloring, then slowly pour one teaspoon of salt into the container. Watch as beautiful colored orbs of oil gently fall to the bottom of the container.
At first, the oil will stay at the top of the container because oil is lighter than water. The key to making the oil fall to the bottom is the salt, it binds to the oil, making it heavier than the water. However, once the salt dissolves in the water, the oil will rise again to the top of the container. Groovy!
Grow Some Crystals
This classic experiment takes few days to complete, but it's well worth the wait.
You'll need some distilled water, salt, a piece of wire or a pipe cleaner, and a glass container. First, heat the distilled water to a point just below boiling. Fill the glass container at least half full with the hot water. Add a lot of salt to the water and stir well.
Make a loop in the wire or pipe cleaner and lower the wire into the mixture. Place the container in a warm spot and wait. After a few days, you should see spectacular salt crystals forming on the loop of the wire.
This experiment works because of the temperature change of the water, and the solubility, the ability to be dissolved, of the salt. As the water cools, the solubility of the salt in the mixture decreases, and the salt precipitates onto the wire to form crystals.
Build a Popsicle Stick Catapult
To build this mini catapult, you'll need at least 10 large popsicle sticks, a bunch of rubber bands, a pair of scissors, and some marshmallows for cannon balls. Marshmallows for canon balls? How dastardly!
Stack eight popsicle sticks, and hold them together with rubber bands at each end. On the two remaining sticks, use the scissors to make a small notch on each side of the stick. Place them together and use a rubber band to hold the sticks together at the notch.
Then, pull the two sticks slightly apart and slide the eight-stick bundle between them. Steady your new catapult with one hand, and use your other hand to place a marshmallow on the top stick. Pull it back and release to fire!
You can also bind a plastic spoon with a rubber band to the top stick to make a bucket for holding your cannon balls. The castle walls will fall!
Make a Prism
You can make a rudimentary prism with just distilled water and clear gelatin. Empty a packet of gelatin into a pot and add only half the amount of water listed in the gelatin package instructions.
Place the pot on the stove, and as the pot warms, stir the gelatin gently to dissolve it. After the gelatin has dissolved, place the mixture into a small container and let it sit for 30 minutes to cool.
Cut the gelatin into squares or prism shapes, which is half of a square or rectangle cut on the diagonal. Shine a flashlight through the gelatin to see the light broken up into its spectral colors. You can also shine a laser pointer through the gelatin to see the light bend.
Create a Whirlpool
You can make a cool whirlpool by using two empty 2-liter soda bottles, a metal washer that has an opening smaller than the mouths of the bottles, and duct tape. Fill one of the 2-liter bottles, 2/3 full of water.
Place the washer on top of the filled bottle, and place the empty bottle upside down on top of the washer. Tape the two bottles together and quickly flip the bottles. You should see a water vortex (aka whirlpool) form as the water from the top bottle flows into the bottom bottle.
The vortex forms because the water spins faster around the edges of the bottle, creating a hole in the middle. This vacuum then fills with air from the bottom bottle, and water from the top bottle flows around it.
Build a Potato Battery
For this experiment, you'll need a potato, a galvanized nail, a copper coin such as a penny, two alligator clip leads with clips on both ends, and a voltmeter.
Galvanized nails have a zinc coating, and they can be purchased at any hardware or home improvement store. Be sure to use a fresh potato because the experiment depends on the juices inside the potato.
Stick the galvanized nail into the potato, making sure that it doesn't go all the way through. About an inch away from the nail, stick the penny.
Connect the penny to the red lead of the voltmeter using one of the alligator clips. Most voltmeters have red and black leads, but if your voltmeter has yellow and black leads, connect the penny to the yellow lead.
Connect the galvanized nail to the black lead of the voltmeter, and make sure both alligator clips are securely attached. Your voltmeter should show a positive reading. If it shows a negative value, simply switch the leads. You've produced electricity from a potato!
You can do all these experiments at home with kids, and they are a wonderful introduction into the worlds of science and engineering.