haha candyman guessed um their lego pneumatics. they do (with enough pressure) hop a little bit with out the battery weight, i put water in the tank i use and it worked faster/better cause the water dosent compress as easily. and to answer sweetdiesel's question it doesn't have a stereo system in it.........yet. to tell you guys the truth i picked up some earbud headphones and one of those little 30 sec song player things and I'm gonna take the lil speaker things outta the headphones and splice um with the player thing and mount um in the body its gonna be.....mini boss