She may be 50, but Madonna can still kick butt — and she proved it during the first night of her Sticky and Sweet tour on Saturday at Cardiff's Millennium Stadium in Wales. The Material Mom wowed the crowd of 40,000 with her dance moves, her 25-year-old's body and on-screen guest appearances from Kanye West, Justin Timberlake and yep, Britney Spears. Britney made her video performance during the song "Human Nature," shimmying in an elevator and signing off with her now-infamous line, "It's Britney, bitch." The Queen of Pop also got political, showing images of Al Gore, Bono and Barack Obama during a video segment. Two hours of nonstop pop ended with a final video message, "Game Over." Madonna heads stateside on October 4, when she comes to New Jersey.