If you don't know the theory behind this, here it is:
Keys are sharp and can scratch paint. When working on a car's engine, you lean over the fender and your pocket, with keys in it, rubs against the paint. Your keys, even through your pants could be able to scratch your car. If you put your keys in your back pocket they're no longer exposed to the paint.
Keys are sharp and can scratch paint. When working on a car's engine, you lean over the fender and your pocket, with keys in it, rubs against the paint. Your keys, even through your pants could be able to scratch your car. If you put your keys in your back pocket they're no longer exposed to the paint.