This invention demonstrates Newton's third law of motion, that for every action there is an equal and opposite reaction. Simply take an empty plastic juice bottle, put a straw in the opening, and seal the opening with modeling clay. Take a slightly larger straw, seal one of its ends with more modeling clay. Then, place the larger straw over the smaller straw. You should feel some resistance. When you're ready to launch, let the straw go and watch it fly.
This invention may sound simple, but it demonstrates Newton's first law. Take a shoe box, and puncture four holes in it with wooden skewer sticks, which will be the axles. Attach a thread bobbin on all four protruding parts of the sticks to make wheels. Take two more sticks and tape them to the box sticking up, and attach a piece of a plastic bag across these sticks to make a sail. The box will stay still when undisturbed, but as soon as you point a fan toward the sail and turn it on, the box will move.
While this invention might not be of use in everyday life, it will demonstrate Newton's first law. Fill a large glass with water, about three-quarters of the way. Take an aluminum pie pan, and place it on the glass. In the center of the pie pan, stand an empty toilet paper roll on its end, and place an egg on top. When you're ready, hit the edge of the pie pan horizontally, making sure to follow through. As the pan is batted away, the egg should fall directly into the cup of water, saving it from breaking, and demonstrating that because of inertia, the egg only moves downward, and not to the side.
This invention will allow you to play practical jokes on your friends, and get your whole class laughing. Use a thumbtack to poke numerous holes in an empty plastic soda bottle. Draw on the bottle over the holes, so they become invisible. Fill the bottle with water, and keep the water running as you screw on the cap (you might want to do this in the sink). The bottle will not leak as long as the cap is on tightly, due to the surface tension of the water molecules. Give the bottle to a friend, and ask him to open it. As he does, he will get soaked, and you will have demonstrated that the water tends to stay still unless it has forces working on it, in this case, gravity and atmospheric pressure.