Super Mario Galaxy 2/Rightside Down Galaxy

Hidden Star: The Great Crate Incinerator
Upon entering the 2D world, proceed until you get to the area where the first switch in the level is located. flip the gravity switch and head left into the Warp Pipe that was previously above you. You will end up in a room with several wooden crates and a fire flower. Transform to Fire Mario and throw fireballs at the crate in the far right corner of the room to reveal a hidden orange Warp Pipe. The gearmo there will ask for your help destroying the all of the crates on the screen. You will have a limited time to destroy all the crates so hurry up. Remember that a single fire ball is capable of passing through multiple crates at a time. Your reward is a shiny Power Star.