Tennis balls are hollow and easy to cut into. This makes them perfect vehicles for intra-office correspondence, hiding precious valuables at the gym, or any other activity that might require ballistic containment.
Just cut a slit into the side of the tennis ball. Cram in your message. Hurl it to your intended recipient.
Cut a slit into the side of the tennis ball. Cram in your cash. Stuff it under some dirty socks in your gym bag next to the Tinactin.