Let's suppose you're thirsty, and the only convenient source of drink nearby is a vending machine, dispensing $1.25 drinks.
You have only $2.00 on you, in bills. The vending machine doesn't give change.
I happen to be walking by, though, and I have a couple of quarters on me. I'll give you my two quarters if you'll give me a dollar.
Alternatively, you could just put $2.00 in the machine and let it keep the change, which would cost you an extra $.25 than if you bought my quarters.
Or you could go without a drink, but you're really thirsty and don't want to do that.
So... How did my $.50 suddenly double in value?