Morph bubble

Morph Bubbles, also known as metamorphosis bubbles, are floating bubbles found in Yoshi's Island in Super Mario World 2: Yoshi's Island, its remake and its sequel Yoshi's Island DS. When a Yoshi touches this, the Yoshi will turn into a different form, usually a vehicle that resembles the Yoshi itself. Some transformations are temporary, while others last until the transformed Yoshi hits a Yoshi Block. If a Yoshi is transformed into a temporary form, it must hit a Yoshi Block to turn back to a normal Yoshi, or pop another Morph Bubble to stay in that transformation a little while longer. Also, if a Yoshi fails to touch another Morph Bubble or a Yoshi Block before the transformation time runs out, he will automatically go back to the exact location where Yoshi last touched the bubble and he will revert to his original Yoshi form.

In Yoshi's New Island, Morph Bubbles are replaced by Whirly Gates.

Transformations
The transformations marked with an asterisk are not in Yoshi's Island DS.
 * Car*
 * Helicopter
 * Mole Tank
 * Submarine
 * Train*
 * Ski Yoshi (Skis)
 * Snowball Yoshi

In Super Mario World 2: Yoshi's Island, there are sprites for three Yoshi forms that do not appear in the game; these include a Yoshi plane, a Yoshi mushroom, and a Yoshi tree.