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

Example Input: Passage: Cynodonts probably gave rise to mammals about 200 million years ago. However, they are not considered to be mammals themselves. In fact, competition with early mammals may have led to their extinction. They went extinct sometime during the Jurassic or Cretaceous Period. Answer: mammals
Example Output: Competition with what emerging animal group may have led to extinction of cynodonts about 200 million years ago?

Example Input: Passage: To harness water power, a stream must be dammed. Narrow valleys are the best for dams. While sitting in the reservoir behind the dam, the water has potential energy. Water is allowed to flow downhill into a large turbine. While flowing downhill, the water has kinetic energy. Kinetic energy makes the turbine spin. The turbine is connected to a generator, which makes electricity. Answer: kinetic energy
Example Output: Water is allowed to flow downhill into a large turbine to harness power. what helps the turbine spin?

Example Input: Passage: You are already very familiar with Earth's gravity. It constantly pulls you toward the center of the planet. It prevents you and everything else on Earth from being flung out into space as the planet spins on its axis. It also pulls objects above the surface, from meteors to skydivers, down to the ground. Gravity between Earth and the moon and between Earth and artificial satellites keeps all these objects circling around Earth. Gravity also keeps Earth moving around the sun. Answer: gravity
Example Output:
What force pulls people and objects toward the center of the earth?