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.

[EX Q]: Question: What do scientists believe are the oldest eukaryotes? Answer: protists
[EX A]: Scientists think that protists are the oldest eukaryotes. They most likely evolved from prokaryotic cells, as explained by the endosymbiotic theory. This theory is well-supported by evidence.

[EX Q]: Question: What do osteoclasts do to bone? Answer: break it down
[EX A]: Under the direction of osteocytes, osteoblasts continuously build up bone, while osteoclasts continuously break down bone. These processes help maintain mineral homeostasis.

[EX Q]: Question: Which system transports oxygens to cells? Answer: circulatory system
[EX A]:
An important function of the circulatory system is transporting oxygen to cells.