This is the perfect prank for your musical friends. Please vote!
Check this one out too!https://www.instructables.com/id/Glue-Stick-Water-Prank/
Step 1: Gather Your Supplies.
1. An upright piano
2. Two small boards that will fit loosely between the hammers and strings of the piano (We don't want to break anything!)
Step 2: Open the Lid of the Piano.
Locate the strings and the hammers to find where to put the boards.
Step 3: Insert Boards.
Gently place the boards so that the hammers will be blocked from hitting the strings.
Step 4: Close the Lid.
When your victim goes to play the piano, the boards keep the hammers from hitting the strings.