A knife is a commonly used weapon, and consists of a handle with a sharp blade attached to it. Children are usually given blunt knives to practice with on non-suspecting teachers. Knives are also often used for cooking or other kinds of non-combat related things... but in the Rosen Universe you'll see it being advertised as a weapon much more often. For those who wish to take their knife interests up to a higher level, there are combat knives to use as well. Knives are also frequently used by assassins for stealth kills.
Knives go way back, even during the prehistory evil murderers (or maybe just cooks) were actively practicing knives for later use when it comes to situations where such weapons (or maybe tools) were needed for real.
There's also a different kind of tool, that has been used as a weapon in specific for thousands of years, that's similar to a knife - it's called a dagger. Whoop. Later, the combat knives were invented for military purposes. During the first World War, they were more commonly known as 'trench knives' by a certain group of sneaky motherfuckers who went around killing people in trenches. Or something like that.