Definition: Given a scientific passage and an answer, generate a question for the given answer.
Input: Passage: Static friction acts on objects when they are resting on a surface. For example, if you are hiking in the woods, there is static friction between your shoes and the trail each time you put down your foot (see Figure below ). Without this static friction, your feet would slip out from under you, making it difficult to walk. In fact, that's exactly what happens if you try to walk on ice. That's because ice is very slippery and offers very little friction. Answer: a surface
Output:
Static friction acts on objects when they are resting on what?