To start, cut off the tips of the matches. (use all boxes.) Next cut a small hole in the tennis ball. Insert the match heads in seven at a time. When finished inserting the match heads, use between 1 and 3 layers of tape to cover the tennis ball. Increasing the thickness of the ball with tape will increase the power of the explosion thanks to a greater build up of internal pressure. Don't use too many layers, however, as covering the tennis ball with too many layers of tape will prevent the tennis ball from exploding. Tada! When you throw the tennis ball the impact will spark the matches creating an explosion!