The masseter, or jaw muscle, is considered the strongest muscle in the human body relative to its size.
Why it is true
This muscle is responsible for closing the jaw during chewing and can exert a surprisingly high bite force, outperforming other muscles in relative strength.
Source: Scientific American
Would you rather be asked first?
This is the reading side of the site, so the answer is in the sentence above — that is the whole point of it. Test yourself works the other way round: the question comes on its own, and the fact and the explanation only appear once you have answered.