On your mouth is a stamp that you tap, strike and slap to create a beat! The stamp looks like a little drum, but you're really just hitting your mouth for fun (the stamp was an excuse to make it seem normal).
As you tap and strike your own mouth, こういち comes dancing into your bathroom. "Hey now, this looks fun, can I try tapping my mouth too?" he asks. "Sure," you say, and offer him a stamp.