If you'd gone down to Shimokitazawa that day — the Saturday before Christmas, around 3 p.m. — you'd have been sure of a big surprise. No, not a teddy bears' picnic, though in Shimokita you never know; instead, among the usual bustling crowds of hipsters, a load of people just stopped moving. For five minutes they stood stock-still, frozen in position, seemingly impervious to the world around them. Then, as abruptly as they had stopped, they started moving again, continuing about their business as if nothing had happened.

This freeze and others like it around the world were inspired by one staged in January 2007 at New York's Grand Central Station by ImprovEverywhere, a network of pranksters based in the Big Apple. On their Web site is posted this mission statement: "We're big believers in 'organized fun.' Our missions are a fun source of entertainment for the participants (and) those who happen to see us live. We get satisfaction from coming up with an awesome idea and making it come to life. In the process we bring excitement to otherwise unexciting locales and give strangers a story they can tell for the rest of their lives. We're out to prove that a prank doesn't have to involve humiliation or embarrassment, it can simply be about making someone laugh, smile or stop to notice the world around them."

While ImprovEverywhere are based in the States, there's plenty such japery going on here in Japan. Chris Wells is a 42-year-old native of Missouri who has been involved with Tokyo Comedy Store since their first show in 1994. As well as helping to run their regular stand-up and improv shows, he also sometimes takes comedy out onto the rails.