“Why is a Raven like a Writing Desk?” “Because they both start with ‘R’.”
This is Lewis Carroll’s classic riddle the Mad Hatter asks to Alice in her titular Adventures in Wonderland. Interestingly, it is never answered in the story and was intended to be a nonsensical riddle that would have no answer. But Carroll did give a fun and clever answer later after years of people asking him. His answer was “Because it can produce a few notes, tho they are very flat; and it is never put with the wrong end in front!” Other writers have suggested clever meta answers as well, such as “Poe wrote on both,” and “Because there is a ‘b’ in both and an ‘n’ in neither.” I like to think that your fake sphinx would both be not very imaginative in his answer, and also clearly a poor speller.