Some scholars have claimed that the very first invention of man was a stick. Still others say no, the first invention was clearly a rock. Frankly, we don't really want to bother to do any research, so we're just going to say we don't know.

What we do know is, shortly after the rock and the stick were invented, some Neolithic genius had the idea to put the two of them together, and thus the knife was born. Although there have been many refinements through the centuries, at its heart the knife still consists of a handle attached to a blade.

Or more precisely, at your heart. For despite the many useful things humans have found to cut with a knife -- food, rope, bedposts -- by far the most popular one has turned out to be each other. Long the weapon of choice for criminals, assassins, and spies, in modern times the advent of better¹ weapons² has reduced the knife in popularity. Although you still wouldn't want to meet one in a dark alley.

¹ e. g. deadlier

² e. g. guns