Shares of the hamburger chain Shake Shack — priced at $21 for today’s IPO — have more than doubled on their first day of trading. Shake Shack has 63 locations and plans to add 10 a year for the foreseeable future.

Danny Meyer’s “modern day roadside” burger joint is now worth more than $1.6 billion — a far cry from its first incarnation as a modest hot dog cart in Manhattan’s Madison Square Park.

Also, the Shake Shack truck will be outside the Stock Exchange today, giving out free burgers and “Shack swag and giveaways” all day (more details).