'Typewriter' is the longest word using only one row of keys.
Why it is true
You can type the word 'typewriter' using only the letters on the top row of a standard QWERTY keyboard.
Source: Linguistics
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.