One student plays on the staff slate and the other student plays on the piano.
Caroline, who plays on the staff slate, says, “William, will you trade me your middle C dolphin for my middle C jellyfish?”
“Okay," and the swap is made between the two students. This makes a clear connection between the note on the staff and which C it is on the piano.
Then it's William's turn to ask, "Caroline, will you trade me your . . . . .”