The principle that this toy is based on the projectile motion and gravity.

I came up with the concept while watching "The Simpsons" on tv where they had flung a cat with a catapult, and at the same time i was thinking about lacrosse. So this toy is kind of a cross between a catapult and lacrosse. The child takes the net with handles, and tosses a ball up, then he catches it by closing the handles around the ball when it gets to him. Then he can fling it at someone or just play catch by himself by ripping the handles apart, thus the catapult. The cost to build the toy will be pretty cheap since the materials are inexpensive and the construction is basically a net with handles attached. It should be pretty durable, and pretty safe since the ball is foam or rubber.