People think of shivs as a perversion of purpose. Spoons, broken glass, pieces of bedframe, stray free-weight bars; functional things forced into a role as weapons, used by terrible people on other terrible people. The guards take pains attempting to control the prisoners, erroneously believing them to be the source of the problem.
They're not. They're just the only ones with enough empty time to hear the silent cries.
Everyday objects know their place, and they yearn for a chance to pay their abusers back, just a little. Everything wants revenge. Everything wants blood.
Everything wants to be a knife.
Anna also suggested "googly eyes" before "shivs," but there already IS a god of googly eyes, and his name is Caspar Babypants. If you need a mood-picker-upper now, just click that link. :-)