Is there anything more rewarding than iconic pop stars sharing their adoration of each other? In honor of World AIDS Day on Dec. 1, Madonna covered Britney Spears' "Toxic" on Instagram. She dedicated the song to both the cause and to Spears, since her birthday happens to be on Dec. 2. And from the sound of Spears' response, the "...Baby One More Time" singer couldn't be more excited about Madonna's gesture.