So a colleague just celebrated his birthday, in the time-honoured manner of giving out cakes and doughnuts to everyone in the room. There's a semi-tradition that one also coyly hints at one's age on these occasions, by providing some sort of riddle. His riddle was simply ‘Rubidium’. (Of course, referring to an actual periodic table or using Google would have been cheating.)

So, after nearly ten years, my A-level chemistry came in useful for something. I was willing to bet it'd never happen. :-)