You use a knife to slice my head and weep beside me when I am dead. What am I?

Why this works

This riddle plays on the dual meanings of "slice" and "weep." When you slice the "head" of an onion, you're literally cutting into it, which releases compounds that irritate your eyes, causing you to "weep." The clever twist here is that while you might think of weeping as a sign of sadness for something dead, in this case, it's actually a humorous reference to the tears caused by cutting the onion. So, the answer is an onion!