Tennis balls get worn out after sometime. This is a great way to reuse them.
1) A knife
2) A tennis ball
1) Use the knife to cut a "slit" in the middle of the ball. *Note: Do not cut all the way through the middle
2) You can use the slit of the tennis ball to put stuff in it, such as your keys, a pen, etc.
*Optional: Decorate your tennis ball by adding eyes, as seen in the pictures.