From the Super Mario Wiki
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Yoshi can eat the bubbles (which are also produced by Barney Bubbles) and then spit them back out to destroy almost any enemy. He can also bounce off of them to reach greater heights, but the bubbles he touches will pop in the process. Once Yoshi blows them out he'll have to lick up another to keep the weapon. The bubbles can only run out if they all pop and the Bubble Dayzee is gone.