Officially, though, a percussion instrument is something specially designed to make a sound when hit. These go from drums:
photo Old Guard Museum, Washington
to the less obvious piano, where the hammers that hit the strings are usually hidden inside the case:
Yamaha piano, photo by Michael Müller-Hildebrand
(Though where that leaves an electric piano or drum-kit I do not know.)
And there are still more percussion instruments around. They may not be musical, but a percussion tool uses the same principle to do its job:
photo of a pneumatic drill by Anthony Appleyard
Or there are percussion caps, which, sadly, don't protect people from blows to the head, but were formerly used as a means of making a gun fire.
I think I'll try to find something gentler, though...does anyone know where I can find a cow with a bell?
photo by Daniel Schwen
Spot the Frippet: something percussive. This word comes from the Latin percutere, to hit.