Given the task definition and input, reply with output. Given a scientific passage and an answer, generate a question for the given answer.

Passage: Figure below shows what happens to waves near shore. As waves move into shallow water, they start to touch the bottom. The base of the waves drag and slow. Soon the waves slow down and pile up. They get steeper and unstable as the top moves faster than the base. When they reach the shore, the waves topple over and break. Answer: topple and break
What happens when waves reach the shore?