I just started a YKTTW for the trope where a sphinx asks you a riddle. To mark the occasion, I'm going to think of a cool riddle.

Okay. Hmm. 

What has a beard but no hair,
Was brown but now is grey,
And is a piece of moldy bread?

Wait, I think I messed up somewhere. This is hard. 

Okay, if I were a sphinx and I wanted to eat some poor schmoe, what would I ask?

...

What is the fifty-first Merseinne Prime?

I'm not sure if I'd make a really bad sphinx or a really good sphinx.