As a kid, Jack had a speech impediment that kept him indoors. He loved trains and maps and taught himself how to program computers by the time he was a teenager.
He found a way to track emergency vehicles and listen in on the dispatch scanner, which was his first exposure to fast and immediate communication (sound familiar?).
"Everyone talked in short bursts of sound about . . . where they're going, what they're doing, where they are," said Jack. Naturally, the dispatch served as inspiration for Twitter.
Later on, Jack hacked his way to his first job at a large dispatch center in New York by exposing a security hole in its website and offering to fix it. They hired him a week later.