Don't worry about your sense of humor; it's perfectly healthy. This is a very low form of joke we call a pun which is most usually a play on the English language's homonymic nature. In this pun the word "head" is the word being played with because "head" usually refers to a part of the body which we would sacrifice a person's head but nobody thinks of sacrifice when they think of lettuce even though the word "head' is used. Jokes are part of the culture and one of the most difficult things to get across cultural boundaries, especially puns.

Humor is one of the most difficult things to translate from one language to another. Yes, you do probably have a sense of humor - just in your own language. The joke here comes from "sacrificing a head of lettuce". If the person was eating meat, they would have to kill some animal. "Sacrificing a head of lettuce" gives the image that the lettuce has feelings and we should feel sad because we had to kill it.

Thank you !!

Depends. "Head"? Geddit?

"Sacrifice" in this example usually means killing an animal. It sounds silly to "sacrifice" vegetables. Yet if the vegetarian niece is refusing meat (an "animal sacrifice"), then maybe she'd be happy if they "slaughtered" a head of lettuce. Trying to pull some extra meaning from "head (of lettuce)" is unnecessary. You sacrifice the whole animal, not just the head.