Use your phone to tag SL (Stockholm public transport) Access cards with graffiti. Draw your own graffiti tag in the app and surreptitiously tag other people's cards in the subway.
Download the TAG Android app to draw and create your tag. Then write your work to any SL Access card. Use the TAG app again to read and view the image stored on a card. Shake the phone to clear the screen.
The image is stored on, what seems to be, some unused storage space on the SL Access cards. There are no guarantees in life, but tests have shown that the tagged cards continue to work as usual in the public transport system.
To use the app, you have to allow unsigned apps and enable NFC on your device.