There's actually such a thing as physical bitcoins. The way they work is each bitcoin has a tamper-proof seal and contains a code that can be entered on the minter's website. When the code is entered, the value of the physical bitcoin is paid to the enterer of the code. Of course, accessing the code destroys the value of the physical bitcoin. However, the fact that it CAN be redeemed for actual bitcoin gives the coin value, just as silver certificates used to function.