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

Q: Passage: As erosion by groundwater continues, the ceiling of a cave may collapse. The rock and soil above it sink into the ground. This forms a sinkhole on the surface. You can see an example of a sinkhole in Figure below . Some sinkholes are big enough to swallow vehicles and buildings. Answer: sinkhole

A: What is formed when the ceiling of an underground cave collapses?
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Q: Passage: As pictured below ( Figure below ), some grazing animals, especially sheep and goats, eat grass right down to the roots. They may even pull the grass entirely out of the ground. Grazing animals can kill the grass. They may thin it out so much that it offers little protection to the soil. If animals are kept in the same place too long, the soil may become completely bare. The bare soil is easily eroded by wind and water. Answer: erosion

A: By pulling out grass by its roots and over-grazing, livestock contribute to what negative process?
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Q: Passage: The eardrum is like the head of a drum. It is a thin membrane stretched tight across the end of the ear canal. The eardrum vibrates when sound waves strike it, and it sends the vibrations on to the middle ear. Answer: sound waves

A:
What hits the eardrum and causes it to vibrate?
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