Super Mario Galaxy 2/Rightside Down Galaxy

Hidden Star: The Great Crate Incinerator
Upon entering the 2D world, flip the gravity switch and head left into the Warp Pipe. Transform to Fire Mario and throw fireballs at the crates to reveal a hidden orange Warp Pipe. The garbage-collecting robot will ask for your help destroying the all of the crates on the screen. Your reward is a shiny Power Star.