Go to Big Boo's House (course 5). Go into the little house on the left. What you will see is an eyeball and a red grating on the floor. Walk onto the red grating and it will start to go down. Walk off the grating while it is still going down and get under it. Mario's head will pop through the top of the elevator. You can do the same thing on Hazy Maze cave (course 6), but duck and hold the duck button and it will look like Mario has totally disappeared.