Detailed Instructions: Given a scientific passage and an answer, generate a question for the given answer.
Problem:Passage: Even though they are very strong, bones can fracture , or break. Fractures can happen at different places on a bone. They are usually caused by excess bending stress on the bone. Bending stress is what causes a pencil to break if you bend it too far. Answer: excessive bending stress
Solution:
What usually causes fractures on a bone?