Nirvana is back together! Well...kinda. Sorta. Dave Grohl and Krist Novoselic will be taking the stage at Wednesday's "12-12-12" benefit for Hurricane Sandy victims. And they'll be joined by Sir Paul McCartney. "I didn't really know who they were," McCartney said of being in the studio with Grohl and Novoselic. "They are saying how good it is to be back together. I said 'Whoa? You guys haven't played together for all that time? And somebody whispered to me 'That's Nirvana. You're Kurt.' I couldn't believe it."