This is basically a modification of Tom Igoe's Monski Pong project from the excellent book Making Things Talk (O'Reilly). When I bought this book from the maker shed, it came with an arduino and a protoshield (both of which you can see at attached by velcro to the top of the ball hat as well as two force sensitive resistors. Unfortunately, the Monski pong in chapter two of MTT uses 2 flex sensor resistors to be embedded in the cuddly monkey (he is the Monski on the book's cover). So, I modified the circuit to use the FSRs instead, these are attached with velcro tape to the inside of the hat's band. The whole thing makes for a challenge as you play pong against yourself using your eyebrows.