Detailed Instructions: Given a scientific passage and an answer, generate a question for the given answer.
Problem:Passage: More than one force may act on an object at the same time. In fact, just about all objects on Earth have at least two forces acting on them at all times. One force is gravity, which pulls objects down toward the center of Earth. The other force is an upward force that may be provided by the ground or other surface. Answer: two
Solution:
How many forces do objects on earth have acting on them at all times?