Why do they call it an Oyster card?

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Tags

1. #metro
2. #called
3. #continues
4. #travel
5. #oyster
6. #pearl
7. #because
8. #couple
9. #earlier
10. #octopus
11. #there
12. #valuable
13. #theme
14. #protects
15. #money
16. #other
17. #something
18. #reasons