Go to Yo'ster Isle. This works best after you've defeated Boshi. Get on the green Yoshi and then ride over to the racetrack. Pick a racing Yoshi and then place yourself in that Yoshi's path. It's better if you do this far ahead of the Yoshi, so you have time to prepare yourself. Jump off of the green Yoshi and then talk to him. He'll ask you if you want to Saddle Up. Say yes, but don't jump on. Wait for the racing Yoshi to get to you and right when it reaches the Green Yoshi so that they overlap, jump on the Green Yoshi. If done correctly, you'll be on the Green Yoshi stuck inside of the racing Yoshi. You won't be able to move around. The racing Yoshi will still be running, but in place. If you do this at just the right moment, the characters' images will look really warped and be inside of each other. To get out, just jump or get off of Yoshi. You can do this with any character that walks around in Yo'ster Isle, but it's easiest with the racing Yoshis because they have a predictable path.
Submitted by: kornyhatemachine