A few scenarios: 

You're a google employee, you have a major project that you're managing, you're stressed, you've barely slept in the past week, you're almost done, and you have to get from one side of the building to the next because your project partner hasn't updated his part of the code and isn't answering his emails. You run through the bridge connecting both parts of the google building. You're not really paying attention, but the bridge's emotion detection system has noticed that you're stressed. It's projecting your emotion above your head in the form of an emoji, and Kayla, your friend and colleague, who is also crossing the bridge, stops you. Her emoji is a happy face. She asks what's wrong and you get to vent a little about your project partner. Kayla lets you know that your partner is currently getting coffee nearby, he'll be back in a minute, and she tells you maybe you should go get a snack too, while you wait. You head back feeling a lot less frustrated.

You're a local, and you walk by the google bridge every day. You like stopping and looking up, the emojis that the bridge projects when an employee walks by are a little amusing. You wonder what your emoji would be at this very moment. That, and last time you stopped, you saw someone with a laughing emoji trip over his shoelaces, the emoji turned red and angry, and the couple of people standing by whose emojis were neutral turned to the laughing emoji. You laughed a little, and walked away.