Behold the Octopus Card.
With just a single swipe of this little card, Hong Kong opens itself up you in the most magical of ways. Most people use it as a rechargeable subway token, but the Octopus card can be used for lots of different purposes. You can pay your gas and water bills (at 7-11, no less), hop aboard any bus, train, tram, cable car, or ferry and catch an easy ride, order a McDonald's value meal, buy your groceries, feed a parking meter, indulge in instant noodles from a vending machine, donate money to various charities, and the list goes on and on. And all with just one little swipe. It's such an efficient system! You can even have a chip implanted in your watch or cell phone and rid yourself of needing to carry the card at all.
More than 95% of people in HK have an Octopus card. Heck, Matt and I have 5 of them. (We stocked up to share with friends and family who were in town, as you get a discount on transportation if you use the card.)
It's pure brilliance. The best part is that if you rig the card in your purse properly, you can just haul your purse over the sensor and never need to take out your card at all. I love it! Can't wait 'til the idea/technology hops the pond and finds its way to San Diego.