Why is this the answer?

The answer is onion because this clever riddle uses a fun and dramatic way to describe the common experience of cutting a very popular kitchen vegetable. Let's look closely at the first part of the riddle: "You use a knife to slice my head." In the kitchen, an onion is a round, strong vegetable that has thin, papery layers on the outside. When a grown-up is getting ready to cook, they often need to cut the onion into smaller pieces, or dice it, to add a yummy flavor to soup or a special dish. The riddle is pretending that the whole, round vegetable is a person, and the action of slicing the onion to prepare it for food is what it calls "slicing my head." This first clue is all about the shape and preparation of the vegetable. The second part of the riddle gives the best and most famous hint: "And weep beside me when I am dead." When we say the onion is "dead," the riddle just means the onion has been cut into pieces and is no longer a whole bulb. Have you ever noticed what happens to people's eyes when they are slicing an onion? Almost every time, their eyes start to get watery, and they begin to tear up. This is the "weeping" the riddle is talking about. It happens because the onion has a wonderful defense system. When you cut into the layers, the onion releases a special chemical into the air, which turns into an invisible gas. This gas floats up and gently tickles the sensitive parts of your eyes. It is not painful, but your eyes react by creating tears to quickly wash the gas away. This automatic action makes it look like you are crying a little river right next to the onion, even though you are not sad at all. This brilliant combination of being cut with a knife and causing tears makes the onion the perfect and only solution to the riddle.