Example:
I went to a store yesterday and bought something that amounted to $9.30, i handed the 16 year old girl cashier a $10.00 bill and thirty cents.
She said to me ''That's too much, sir''.
I explained that i wanted a dollar back in change so i could use the car wash next door. (We have dollar coins here in Canada).
She gave me a surprised look, and i told her to enter the amount i gave her in the cash register anyway.
She did so and the change came out to exactly $1.00.
She couldn't believe that i could do that calculation in my head so fast!