Given the task definition and input, reply with output. Given a scientific passage and an answer, generate a question for the given answer.

Passage: An addition reaction is a reaction in which an atom or molecule is added to an unsaturated molecule, making a single product . An addition reaction can be thought of as adding a molecule across the double or triple bond of an alkene or alkyne. Addition reactions are useful ways to introduce a new functional group into an organic molecule. Answer: an addition reaction
What is a reaction in which an atom or molecule is added to an unsaturated molecule, making a single product?