Given a scientific passage and an answer, generate a question for the given answer.
Q: Passage: Experiments are sometimes hard or even impossible to do. For example, a scientist who is studying an extinct animal cannot experiment with the animal because it no longer exists. The scientist must rely instead on evidence in the natural world, such as fossils that the extinct animal left behind. Answer: experiments
A:
What can be hard or even impossible to perform in a scientific sense?