When you have Yoshi as your steed, have him shove different shells into his mouth with his tongue. If you have him grab a red shell, he'll spit fireballs if you have Yoshi spit up. If Yoshi gets a blue shell, he'll sprout wings. Tap A repeatedly, and you'll be able to fly in the air. If you have Yoshi grab a yellow shell, he'll kick up sand clouds every time you come down from a jump. Grabbing a green shell will not help you in any way except ammunition.