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.

[Q]: Question: The outer layer of what anatomical structure is so tough that it keeps out most pathogens? Answer: skin
[A]: This drawing shows that the skin has many layers. The outer layer is so tough that it keeps out most pathogens.

[Q]: Question: The central nervous system is composed of the brain and what else? Answer: the spinal cord
[A]: User:Grm_wnr/Wikimedia Commons, modified by CK-12 Foundation. The central nervous system is composed of the brain and the spinal cord . Public Domain.

[Q]: Question: Flatworms have a concentration of nerve tissue in the head end, which was a major step in the evolution of what organ? Answer: brain
[A]:
Flatworms have a concentration of nerve tissue in the head end. This was a major step in the evolution of a brain. It was also needed for bilateral symmetry.