Q: Given a scientific passage and an answer, generate a question for the given answer.
Passage: Regardless of what gravity is a force between masses or the result of curves in space and time the effects of gravity on motion are well known. You already know that gravity causes objects to fall down to the ground. Gravity affects the motion of objects in other ways as well. Answer: gravity
A:
What is the force that causes objects to fall down to the ground?