SDS

This little magic trick is actually the least of what this billboard can do. This billboard and many other digital billboards are watching you. O'Brien can tell which cellphones are near his billboards. The billboards themselves have sensors, and he also buys location data from cellphone carriers. He syncs that with data he buys about who owns those cellphones, from places like search engines, data aggregators, apps.

So he knows a lot about who sees the ads on his billboards. He knows age, race, gender, credit scores, lifestyle preferences. He knows what you've been doing before; sometimes he even knows what you've been doing before, during and after you look at his billboard.