Building a boat for science class can be used to illustrate the ideas of displacement and propulsion. Displacement is what makes a boat float. To float, the weight of the boat in the water (and the air in the boat) has to weigh the same as the water it pushes out of the way. When the weight of the boat is the same as the water that it pushes out of the way, the boat will float. Propulsion is what makes the boat move forward. Save and rinse out a milk carton from your lunch. You may need permission to take the carton out of the lunch room. Locate a clear plastic tub or empty glass bowl. You'll float your milk-carton boat in this bowl or tub. Cut the carton in half from top to bottom. The place you want to cut is down through the middle of the "roof" so that the open side is removed. Keep cutting down the sides of the carton and all the way around. When you look at the carton so the open side you cut is facing you, the carton should form a pentagon. Turn the carton over so that it rests on the largest (uncut) side. This is your boat. Add a rudder. Cut a shark-fin shape from the leftover milk carton and glue it using the rubber cement to the bottom of the boat so that it moves in a straight line. This fin is called a rudder. If the rudder is on crooked or the ship is not perfectly symmetrical in the water, the ship will tend to go in circles. Make a sail. Propel the boat using wind. Stick a wad of gum in the inside of the boat and a straw you cut in half into the gum in the bottom of the boat. Use a more gum to stick on a paper sail to the straw. Make the sail two inches square. Rocket-propel the boat by sticking the open end of a balloon on a straw with a wad of gum. Stick the other end out a hole you cut in the back of the boat at the center of the corner that is the deepest in the water. Use more gum to seal the gap around the hole. Inflate the balloon and hold the straw pinched so that the air does not escape until the boat is in the water. Making a Paddle Wheel- Milk carton- Plastic tub or clear glass bowl- Safety scissors- Rubber band- Rubber cement- Five Styrofoam peanutsPropulsion comes from Newton's laws. The third law states that for every action, there is an equal and opposite reaction. Every time you propel the boat, think about questions that your teacher will ask. For Newton's third law, the teacher will likely ask about how the boat and water are pushing against each other. Look in your textbook for more clues about what your teacher may ask. Only build this under adult supervision. Never put anything in your mouth that is not clean. After the gum comes out of your mouth, clean your hands and do not chew the gum again or allow your friends to do so. Do not get gum on hair, clothes, carpet, or anywhere else it can do damage. Be careful using scissors and only use safety scissors. At the back of the boat, cut in a square that is 1 inch on all sides. Use gum to attach Styrofoam peanuts inside the boat around the edges of the carton so that the boat does not sink. Put a rubber band around the back end of the boat. Take a piece of carton that you cut off earlier. Flatten the piece of carton and cut it so that it is 3/4-inch wide by 3/4-inch deep. Stick this piece into the rubber band and use a paper clip to hold the rubber band onto the cardboard. With the boat facing left, twist the cardboard clockwise until the cardboard of the boat starts to give (but don't wait for the cardboard to collapse). Place the boat into the water and let go of the twisted paddle wheel. Things You'll Need- sailboat image by brelsbil from Fotolia.com