Detailed Instructions: Given a scientific passage and an answer, generate a question for the given answer.
Problem:Passage: Sometimes cancer cells break away from a tumor. If they enter the bloodstream, they are carried throughout the body. Then, the cells may start growing in other tissues. This is usually how cancer spreads from one part of the body to another. Once this happens, cancer is very hard to stop or control. Answer: bloodstream
Solution:
How do cancer cells typically spread from one part of the body to another?