Detailed Instructions: Given a scientific passage and an answer, generate a question for the given answer.
Q: Passage: Simple diffusion occurs when a substance diffuses through a cell membrane without any help from other molecules. The substance simply passes through tiny spaces in the membrane. It moves from the side of the membrane where it is more concentrated to the side where it is less concentrated. You can see how this happens in Figure below . Answer: simple diffusion
A:
What occurs when a substance diffuses through a cell membrane without any help from other molecules?