The shortest reigning monarch ruled for just 20 minutes.
Why it is true
In 1830, Louis XIX of France technically became King after his father abdicated, but he abdicated himself just 20 minutes later during the July Revolution.
Source: History
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.