Customer: I would like a coke
Salesman: That's will be 10$
C: What? 10$ - that's a ripoff.
S: Well, if you buy two cokes it's only 1$ But you have to drink the 2nd coke on Saturday.
C: Well, just give me the two cokes and I'll throw out the other one.
S: You can't do that - if you don't drink it on Saturday we will have to charge you 10$
C: How would you know whether I had the coke?
S: We have our ways.