From the Super Mario Wiki
The Chuckola Reserve is located deep in Chucklehuck Woods; it is a form of Chuckola Cola. In Mario & Luigi: Superstar Saga, Mario and Luigi have to find the Chuckola Reserve to cure Queen Bean, who swallows a Belly Blech worm, morphing her into a monster, a condition that can be reversed only by drinking the beverage.
Mario and Luigi eventually find the cola inside Mother Chuckalola, along with Bubbles, the founder of Chateau de Chucklehuck. Bubbles had worked on the soda for one thousand years, telling it joke after joke so it would mature. However, Bubbles is so protective of his Chuckola Reserve that he transforms it into a monster, the Chuckolator, upon the brothers' approach, which then engages the duo in battle. After the Chuckolator's defeat, Bubbles sends the now-inert cola and the Mario Bros. underground. Mario lands in the barrel containing the soda, filling him up with it. Luigi ventures through the area alone before riding the barrel with Mario in it back to Beanbean Castle and using the cola within to cure Queen Bean.
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