In Bowser's Castle, as any player with any kart, go to where there's a thwomp and wait. Have a second person get a spiny shell and fire it far from where he is. Now, have the first person get smashed by the thwomp when the spiny shell is about to hit him. If you can time it right, the first person will get hit with the spiny shell when the thwomp is about to raise up. If done correctly, he will be stuck to the bottom of the thwomp and partially through it. He won't fall off it until he gets large again. If the spiny shell doesn't hit you when the thwomp is on you (so the thwomp would be raised up), you will float above the thwomp for a couple seconds until you get large again.
Submitted by: Derek