Given a scientific question and its correct answer, generate supporting facts for the answer. This supporting fact can be an explanation for the answer to the given question.
--------
Question: Question: Fossils provide a window into the past. they are evidence for what? Answer: evolution

Answer: Fossils provide a window into the past. They are evidence for evolution.

Question: Question: The hamstrings flex the leg, whereas the quadriceps femoris have what effect? Answer: extend

Answer: The hamstrings flex the leg, whereas the quadriceps femoris extend it.

Question: Question: What are groups of young stars loosely held together by gravity called? Answer: open clusters

Answer:
Open clusters are groups of young stars loosely held together by gravity.