Pennies are the pinnacle of cheap science project materials.

Cool tricks, like popping a penny off a frozen bottle or spinning a penny inside of a balloon, can be easily turned into science fair projects by carefully changing one variable and determining if it affects the results of the "trick." Fourth grade students can understand scientific concepts such as centripetal force and the expansion of a gas when heated and are ready to test out these principals in real life experimentation.

### Spinning Penny: A Study in Centripetal Force

Centripetal force will keep a penny spinning inside a balloon.

Stretch the opening of a clear balloon and press a penny through the mouth.

Blow up the balloon. Be careful to not over-inflate. Tie the balloon.

Hold the balloon from the tied end, so that the tie rests in your palm and your fingers and thumb gently grip the balloon.

Turn your palm facing down and swirl in a circular motion until the penny rolls smoothly around the balloon. Once it gets spinning, put your other hand at the bottom of the balloon and hold it still. It should keep spinning for 30 seconds or more.

Time and record how long the penny keeps spinning.

Turn this fun activity into a science experiment by adding one variable -- such as size of balloon or type of coin-- and complete tests to determine if the time the penny spins changes.

### Cool Coin Launcher

Place bottle -- with lid -- into the freezer. Wait 1 hour.

Wrap a penny in a piece of tissue and wet it with water.

Open the bottle -- leaving it in the freezer if possible -- and place the tissue wrapped penny on the bottle opening. Press so it is sealed.

Return the bottle to the freezer for approximately 30 minutes until the tissue freezes.

Remove the bottle from the freezer and warm it with your hands. As the air inside the bottle heats up, the penny should pop right off the top.

Turn this into an experiment by altering one variable such as temperature -- put in refrigerator instead, size of bottle, type of coin or different amount of tissue to determine if changing that variable will affect how the penny pops.