Air? That will be 50c please

I had a good laugh at myself the other day. I was trying to put some air in the car tyres at a petrol station and couldn't figure out why the pressure wasn't going up. It didn't even occur to me until I had been around and tried to fill every tyre that I might be required to pay for the privilege! It was pretty embarrassing because there was a guy waiting behind me who saw me visit every tyre, scrabble in the ashtray for some change, and go back to every tyre for round 2 :)