The Oyster card is 15 years old. You may already know some more common facts like how an Oyster has no expiry date, but here's some we think you might not have pegged just yet.
1. There's a lot of money left on them
According to figures from TfL, there's more than £321m of balances and deposits sitting on Oyster cards that haven't been used for at least one year.
This is partly due to the number of people switching over to using their contactless bank cards or mobile payment technology for travelling on the network. Just five years ago, the total was only at £47m.
Since it was launched 15 years ago, more than 100m people have used an Oyster card to travel.
2. It almost wasn't called an Oyster
Before TfL settled on Oyster, two other names were shortlisted as potential monikers.
Perhaps in an alternate universe, Londoners could be using Gem or Pulse cards to pay for their transport.
3. Tech trials started almost 12 years before the card was born
Development of TfL's smartcard system for ticketing began back in 1991, with the first trials being carried out on the 212 bus in Walthamstow.
A more widespread trial followed in Harrow in 1994, before Oyster was officially launched to the public on 30 June 2003.
4. Do you own a special edition Oyster?
A number of limited edition Oyster cards have been designed over the years to commemorate special events in British and London history.
These have included the 2012 Olympic and Paralympic Games, the 2011 royal wedding between the Duke and Duchess of Cambridge, the Queen's Diamond Jubilee, and the 150th anniversary of the London Underground.
5. Contactless travel is speeding up time
The Oystercard system can handle 40 people per minute passing through a set of Tube barriers, which is 15 more than those with paper tickets.
If you're on a bus, boarding is now an average of three times faster than the old days when buses still accepted cash.
This article originally appeared on City A.M.