The riddle
What English Word Has Three Double Letters in a Row?
Spelling it out
Write bookkeeper one letter at a time: b, o, o, k, k, e, e, p, e, r. The pairs oo, kk and ee sit side by side with no letter between them. Merriam-Webster defines a bookkeeper as a person who records the business transactions of a company. The word is two plain words joined, book and keeper, and each keeps its own double: book has the oo, and keeper has the ee. The kk appears where the two halves meet.
Bookkeeping fits too
The related word bookkeeping has the same three pairs, oo, kk and ee, because it swaps only the ending. Plural bookkeepers works as well. A scan of the standard Unix word list, which holds hundreds of thousands of entries, finds only bookkeeper, bookkeeping and the rare subbookkeeper with three pairs in a row, so the riddle is safe to ask even of a person with a large vocabulary.
Words that almost qualify
Committee has three doubled letters, mm, tt and ee, but they are separated by other letters, so they do not sit in a row. Mississippi has ss, ss and pp, again with single letters between them. Both are common wrong answers. The rule that matters is that the pairs touch each other, one right after another.
A game with the same idea
A different word game asks players to work out a hidden rule: see what can go through the green glass door.