Instructions: Given a scientific passage and an answer, generate a question for the given answer.
Input: Passage: Not long after Wegener's death, scientists recognized that there is convection in the mantle. Deeper material is hotter and so it rises. Near the surface, it becomes cooler and denser so it sinks. This creates a convection cell in the mantle. Answer: rises then sinks
Output:
How does deeper material cause convection in the mantle?