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. Collect all the feathers, then 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 deformed spinning thing. If you spit it between the two pipes, you can experiment a little. You'll notice that Mario cannot stomp it, nor can it hurt him or any enemies. But if you make Yoshi eat it, you get a Cape Feather.