On second thought, if you have no idea what we're talking about, please stay: Let us explain how the Grateful Dead, a '60s-era rock band whose main vocalist and guitarist died in 1995, is about to sell out a three-day stand at Soldier Field in July. There are 180,000 tickets to be sold, but requests have already exceeded 300,000. For a band that's missing its leader and hasn't performed in 20 years.