Make sure you have the Green and Red Switch Palaces cleared, and bring Yoshi and Caped Mario into Star World 4. Have Yoshi eat the Key, then run and fly to your left, hitting all the Green [!] Blocks. Next, get up on the platform with the Koopa that kicks a shell. Spit out the Key and it you'll see that it turned into a shell-type object that cycles between a flopping Cheep-Cheep and Yellow Koopa without a shell. Notice that Mario cannot stomp it, nor can it hurt him or any enemies. Have Yoshi eat it and you'll get a Cape Feather.