Some of my colleagues were giving me the rasberries about bicycle commuting. They said, "ya, but one person's not going to make that much of a difference by not driving to work every day".

I replied, "imagine if we parked your car right here in the middle of this office and ran it for the amount of time you will drive to and from work today. Even at idle, the exhaust would kill all of us. You contribute that much poison gas to the air every day!"

It proves to be a convincing argument.