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

Passage: Earth and Moon orbit each other. This Earth-Moon system orbits the Sun in a regular path ( Figure below ). Gravity is the force of attraction between all objects. Gravity keeps the Earth and Moon in their orbits. Earth's gravity pulls the Moon toward Earth's center. Without gravity, the Moon would continue moving in a straight line off into space. Answer: gravity
Output:
What keeps the earth orbiting the sun?