This easy to build boat uses resources you should be able to find around the house. Remember Newton's III Law, "For every action, there is an equal and opposite reaction". So, as the flipper pushes against the water, the water pushes back and the boat is propelled forward.
You will need:
Small water bottle and cap or recycled plastic bottle
flipper (plastic gift card works great)
1) Fill your water bottle with a little bit of water to give it some weight. Tape the two pencils to both sides of the water bottle. You'll want to leave at least 2/3 of the pencil length hanging down from the bottom side of the bottlle.
2) Place a rubber band between the two pencils.
3) Make a flipper by cutting down an old plastic gift card or solicitation credit card to size. You want the flipper to slide in the rubber band and not touch the pencils or the bottle. Twist the flipper over and over in the rubber band. Hold onto the flipper so it doesn't unwind yet.
4) Place the boat into the water and release the flipper.
5) As the flipper unwinds and pushes against the water, the boat is propelled.