A knife is a sharp tool that's used for slicing or cutting. We suggest using a knife as a kitchen tool instead of as a weapon.
A bread knife has a long, serrated edge, and the blade of a pocket knife folds into its handle so you can safely carry it in your pocket. Surgeons use very small knives to make delicate cuts during operations, and various kinds of knives are sometimes used as weapons. To knife, in fact, is to stab or cut with a knife, almost always in a violent way.