Make a simple cannon out of some tube and a vacuum cleaner.
Step 1: Find Materials
Find a length of tube. PVC or cardboard tube will work. Make sure you will be able to hook up a vacuum to somewhere on the tube, but not the end holes. Then find any ball that is close to the inside diameter of the tube.
Step 2: Attach the vacuum
Cut a hole somewhere near the end of the tube. Attach the vacuum to the hole. I chose to use another length of PVC to run from the PVC barrel to the vacuum hose. It is important to make sure that the hose won't get in the path of the ball traveling up the tube, so basically the hose can't be protruding to far into the tube. When attaching, make sure it is pretty air tight. Wrap it in duct tape a bunch and that should seal it.
Step 3: Test It!
Place the tube on a horizontal or angled surface. Put the ball in the tube, then press something (newspaper, plastic, etc.) on each end. Start the vacuum and it should hold the seals on the end. Remove one of the seals and the ball should fly out the other side. Depending on how you did it, the ball can have a lot of velocity, so be careful.
Play with the different types of tube and the length and diameter of the tube. Some will work better than others. Change the type of projectile, too. For some fun, try shooting marshmallows or mini marshmallows. There are endless possibilities with this project.