At the beginning of the episode, Sheldon is using a ping pong ball-shooting gun to do a gravity experiment. He has the gun clamped in a bench vise, aimed up at a doll (The Joker from Batman) hanging from a ceiling rafter. A close-up shot clearly shows the gun pointing upwards as Sheldon squeezes the trigger. Then a wide shot shows the gun clamped horizontally as it shoots the ping pong ball at the doll as the doll is falling to the floor. The next shot shows the gun, clamped in the bench vise, pointing up in the air again.