The human brain is 75% water.
Why it is true
Water is essential for electrical impulses and chemical reactions, meaning dehydration directly affects reasoning.
Source: USGS
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.