caption: Slide 2 of 9: When you think about it, the ``room keys'' you get when you check into a hotel really aren't keys at all. More likely than not, the desk clerk gives you a plastic card that you tap or swipe to access your room. But hotels did used to give guests regular old metal keys. Before the advent of plastic key cards in the 1970s, travelers unlocked their rooms with traditional metal keys attached to ``key fobs.'' These large plastic tags were useful in a couple different ways: They usually had the name and address of the hotel, as well as the guest's room number, printed right on them. And if you happened to forget to drop your key off after you left the hotel, the key fobs contained a small stamp reading ``Guaranteed Postage Paid.'' You could just place the key in a mailbox and it would make its way back to the hotel. It was less costly for hotels to pay for the postage than it was for them to replace the key. Learn the secrets hotels won't tell you.