1. No one really knows (to truth about or answer to something). A: "What does the future hold, now that you've left the music industry?" B: "Who's to say, really? I've always been interested in writing a novel, so I may pursue that." A: "Why on earth have they decided to go this route with the new software?" B: "Who's to say what convoluted analytics they employ to come up with their new designs?"

2. According to whose authority? Who's to say I can't take another holiday already? I have the days to take, and it doesn't conflict with anyone else in the office. I know we've had a tough season so far, but who's to say we can't go all the way and win the championship?