This is similar to the Double Yoshi glitch, but the following method will create many more Yoshis. In Donut Plains 1, at the spot where a Super Koopa flies by a Question Block that contains Yoshi, hit the block and get hit by the Super Koopa at the same time. You may have to use your cape to hit the block as you get hit by the enemy. Next, try to jump onto Yoshi, and Mario will bounce in mid-air above Yoshi before finally landing on him. Yoshi's "zip-zoo" sound effect will play on each bounce, creating a stuttering sound like "z-z-z-z-z-zip-zoo." The really cool part is that you can lose Yoshi down a pit, then return to his block to find another Yoshi waiting!