What You Do:
- Put an empty plastic bottle (from water or soda) in the freezer, with the lid off.
- Take the bottle out of the freezer after one hour. Get the penny wet and slide it around the top of the bottle, until it sticks there. Put the penny and the bottle back in the freezer for one more hour.
- Take the bottle out of the freezer, and hold it in both of your hands being careful not to squeeze the bottle too tightly. Your hands should start to warm up the bottle.
- After a few minutes of holding the bottle in your hands, the penny should pop up into the air!
When air warms up, it takes up more room (expands). When the air inside the bottle got warmer, it expanded - but it had nowhere to go! So, the penny was pushed out of the way by the air. In a rocket engine, two different kinds of fuels are heated and mixed together to make a lot of gas, and it expands very quickly. Just like in this experiment, the pressurized gas wants to escape. In a rocket, it's directed out through a nozzle and hits the ground with enough force to push the rocket up into the air!