Ancient Romans used human urine as mouthwash and crushed mouse brains as toothpaste to whiten their teeth.
Why it is true
The ammonia in urine acted as an effective cleaning agent, while medical recipes of the time included bizarre ingredients for oral hygiene.
Source: Ancient Hygiene
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.