From the Super Mario Wiki
Piranha Plorps are Piranha Plants that have a head that resembles a human heart, and a vein as a stalk. They are found inside Bowser's body in Mario & Luigi: Bowser's Inside Story, encountered in the Energy Hold. They spit poison at Mario and Luigi. One of its attacks includes fogging the bottom screen and spitting red and green blobs of poison at the top screen. The color of these blobs indicates which Bro. will get hit: red for Mario, and green for Luigi. The attack can be countered by using the hammer. The Piranha Plorp will also try spitting a purple stream of poison at the bros. The number of lumps that rise up its throat indicates how many passes the jet of goo will make. This attack is avoided by jumping. If Mario and Luigi are hit by this attack, there is a high possibility of becoming Poisoned. The fastest way to defeat one is by using Fire Flowers, as they are weak to fire.
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