Everyone has there own technique some people are rubs some scratches but everyone has a technique that they use when the coin does not work in the vending machine. You know the scenario you put a coin into the vending machine and it pops out. You try again thinking you just got the wrong velocity in so you put it in harder and it does not work. You rub it or scratch it and then amazingly the machine takes the money.  So how does that happen.

I am not quiet sure but here is a few ideas.

Firstly there is varying methods of how vending machines check the money. Some weigh the money, some check the size and some check the varying electrical and magnetic properties of the coins. For instance each coin will have its own weight and size compared to others and also a different resistance. While the first two or obvious the latter is not so much. Basically as each coin has a different composition it will have a different electrical resistance due to the presence of the various metals in the coin copper, nickel etc. So by putting a current across it it can be checked what resistance it has hence what it is. Also in some vending machines the coins go past electrical coils and will distribute the magnetic field differently depending the metals in the coin.

So how does the rubbing of the coins help. Well I think of two reasons. One it does nothing and is a kind of placebo and you just put the coin in correctly so it does not over or under shoot the mechanism. Or 2 the act of rubbing removes something off the coin that is altering the properties that the machine tries to measure.  So next time I am buying something in the vending machine and this happens I will thoroughly experiment and give you an answer.  If anyone has any other ideas let me know.