Two Bits

The caster knocks in a specific, oddly compulsive rhythm (5 knocks) which makes the target compelled to finish the melody (2 knocks) any way they can. 

The target must make a wisdom saving throw. On a failure, they must respond to the spell with two 'beats' any way they can via knocking, tapping the floor, clicking of jaws, etc. If they are physically unable to return the knock, they take 1d4 psychic damage and feel pretty foolish. Targets who cannot hear or are deafened are immune.

"Shave and a haircut, two bits~"