Melting Oysters

An Oyster Card is the London Transport System's electronic ticket.  You load them and the use it like a debit card for any public transport in the city (tube, train, bus or boat).  You then touch them to the reader at each station to enter the tube or bus or whatever you're trying to travel on.

And thankfully, they still work after you forget to check your husband's pockets before doing laundry...