tap on

tap someone or something on something

to make one or more light blows on some part of someone or something. Someone tapped me on the shoulder, and I turned around to see who it was. I tapped the drum on the top to find its pitch.
See also: on, tap

tap on something

to make one or more light blows on something. Who is that tapping on my windowpane? I wish you would stop tapping on the tabletop.
See also: on, tap