Micro NFC tags
A few years back, I started putting micro NFC tags in my 3D Eno’s. NFC stands for near field communication, and all modern phones are capable of reading NFC tags. Anything can be encoded into the tag, and read by your phone by holding your phone closeby. The encoded message is generally pretty short (especially on the cheap tags I purchased), but it’s long enough of a message to hold a web address. In the experiment on this page, the NFC tags are causing the phone to go to a specific URL.