Not a stupid question, but a VERY irritating statement when I have just settled myself into an empty seat (bus, meeting, cinema etc.) and the person seated next to me says "Sorry, someone is sitting there".

Just once, I got up, turned round and addressed this imaginery person with the words "I do apologise for sitting on you". Surprisingly little reaction was shown.

Why did I get up? If there is a next time I shall say in a very loud voice "YOU'LL HAVE TO SPEAK UP, I'VE LEFT MY HEARING AID AT HOME" and continue in that vein until they get fed up and leave (or someone much larger and uglier than me turns up to claim their seat).