Great exercise and stress relief at little to no cost to anyone.

Also:
Rock slings are one of the oldest weapons known to man (some estimate at least 10,000 yrs). They are extremely simple to make out of raw materials, and it takes little time to do so. Ammo is found nearly everywhere you look (it is versatile enough to sling a lot more than just rocks). The distance record stands at nearly one third of a mile. Velocities can exceed 360 fps. It is virtually weightless, takes up only a tiny space in your pocket and creates very little noise when used. The stopping power of a large rock can be deadly when thrown by hand and becomes over 4x as deadly when used in a sling.