I've just picked up on this discussion on Textism concerning what you think about when you're not thinking of anything else. There are some great answers: my favourite was, "What would happen if I started a Slinky going down the up esclator?", and the scariest was, "Sex and viral vector purification via HPLC".
On the bus coming home yesterday, my thinking about nothing ran thusly. "Why has that guy got dark hair and a grey moustache? Does he dye his hair? I don't see any grey roots, and why wouldn't he dye his âtache too? Is it possible for just your facial hair to go grey? That's weird. Surely your facial hair is younger than your head hair. Idiot — they both get replaced every few days. Maybe he leaves it so that women will stare at him and wonder why his hair is dark and his âtache is grey. Must stop staring at him."