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

Passage: Every surface has microscopic bumps, lumps, and imperfections, emphasized as in the image above. If we try to slide the top block over the lower block, there will be numerous collisions as the bumps impact each other. The forward motion causes the collisions with bumps which then exert a force in opposite way the block is moving. The force of friction always opposes whatever motion is causing the friction. Answer: friction
Output:
What force is caused by microscopic bumps, lumps, and imperfections colliding?