In the fortress in Grass Land, World 1, there is a secret room where there is a chest. That chest contains a legendary Warp Whistle. There are only three of them in the game. When you get to the part of the fortress where there is a Dry Bones, a Koopa zombie. Hit the ? block as Super Mario or Luigi, and get the leaf the pops out from it. Run from where the door is, and fly up until you hit terra firma. When you hit a dead end, press up on the controller. You'll fall from a height and a chest will appear. Run through the chest, and you will get a warp whistle.