You know a mailbox when you see one. (They’re those blue hunks of metal bolted to the sidewalk with the creaky flaps that go reeeeaaaaaallllk when you pull them open.) But what about the dark ...

What are those green mailboxes on the streets of New York City? In the latest Cities 101 we dig into these boxes, known as relay boxes.

Photo via UltraClay's Flickr. Have you walked by those green mailboxes before and wondered what their purpose was, but by the time you were near the Google Machine the question had vanished?