Given a scientific passage and an answer, generate a question for the given answer.

Input: Consider Input: Passage: Muscles cannot contract on their own. They need a stimulus from a nerve cell to "tell" them to contract. Let's say you decide to raise your hand in class. Your brain sends electrical messages to nerve cells, called motor neurons, in your arm and shoulder. The motor neurons, in turn, stimulate muscle fibers in your arm and shoulder to contract, causing your arm to rise. Involuntary contractions of cardiac and smooth muscles are also controlled by nerves. Answer: stimulus

Output: What do muscles need to contract?

Input: Consider Input: Passage: Fats may be either saturated or unsaturated. A saturated fat is a fat that consists of triglycerides whose carbon chains consist entirely of carbon-carbon single bonds. Therefore, the carbon chains are saturated with the maximum number of hydrogen atoms possible. An unsaturated fat is a fat that consists of triglycerides whose carbon chains contain one or more carbon-carbon double bonds . A fat with one double bond is called monounsaturated, while a fat with multiple double bonds is called polyunsaturated (see Figure below ). Answer: saturated and unsaturated

Output: What are the two types of fats?

Input: Consider Input: Passage: A: Like the strings of cello, anything that vibrates produces waves of energy that travel through matter. For example, when you throw a pebble into a pond, waves of energy travel from the pebble through the water in all directions. Like an incandescent light bulb, anything that glows consists of matter that produces light energy. For example, fireflies use chemicals to produce light energy. Like a moving tennis racket, anything that moves has energy because it is moving, including your eyes as they read this sentence. Answer: energy
Output: Like the strings of cello, anything that vibrates produces waves of what?