We know all about what happens when "2 Become 1." But what about when five become three?The Spice Girls are marking their 20th anniversary by embarking on a reunion tour, the group announced in a YouTube clip yesterday. There are just a couple of catches: One, they'll be carrying on without Mel C (Sporty Spice) and Victoria Beckham (Posh Spice), both of whom chose not to rehash their pop star days. Two, they're no longer the Spice Girls.You read that right. Mel Brown (Scary Spice), Emma Bunton (Baby Spice), and Geri Horner (Ginger Spice) will henceforth be known as GEM, a moniker linked to their initials."We've been thinking... We have the best fans in the entire world," the ladies say in the video posted last night."You've stood by us for 20 years, and we wanna say a big thank you. We've had our ups and downs, but you've always been there. And we wanna celebrate and have a party. And when we do... You're all invited!"The video ends with a note that more news is coming soon. In the meantime, we'll be pouring one out for Posh and Sporty.