Ever need to hold down a key on your keyboard for a long time, but don't want to sit there holding it? (I sometimes need that while playing games, eg. when there's a spot where my character can auto-level by just standing and attacking.) Well, this little thing will hold it for you! With the right number of pennies (15 for me) this will hold the key down when pressed, but allow it to stay up if set down lightly.
Keyboards differ; some might need a slightly wider or narrower weight.