Oyster Cards

Visitor Oyster Cards for the tube in London can be bought here in the US already loaded with 30 BPS (british pound sterling) for use on the tube. But here is the question: why are they called "Oyster" cards?

Waiting for July 8th!

We have tickets, we have tickets...The Louvre, Paris Metro, Accademia in Florence...have to buy Oyster cards for the tube...

We even have reservations for dinner in Barcelona at Con Gracia...a fusion restaurant, and we are dining at the elegant hour of 2200.

Now the question...what to pack!