What fruit do twins love?

This joke is a pun, which is a form of humor based on word play that exploits the different possible meanings of a word or the fact that there are words that sound alike but have different meanings. The humor in this joke stems from the dual meaning of the word “pear.”
The setup asks what fruit twins love, leading the listener to expect an answer that relates specifically to twins. The punchline is “a pear,” which is a play on words. The word “pear” is a type of fruit, but it sounds exactly like “pair,” a word commonly used to denote a set of two items that are similar or identical, just like twins. The humor comes from the pun on “pear/pair,” linking the concept of twins (who are a pair of siblings) with the fruit “pear,” which sounds the same.