This tutorial video demonstrates how to make a simple cross bow. Materials required are two sticks, elastics big and small, tape and the tube of a pen (just take of the top and the bottom of the pen).
First tie both the ends of the sticks with elastics. Get the tube of the pen and put it in the middle of the two sticks. Tie extra elastic round the pen tube for support. Now take big elastic, or you can tie some elastic together to make it a bigger one. Tie both ends to the two ends of the sticks tied together. Put some tape to the middle of this elastic to make a handle to place the arrow and hold. A very simple cross bow is ready.