Invention of the Plunger
Basically, one day Bowser went to the bathroom, and he couldn't flush the poop. Mario was his only hope, and thus Mario created the Plunger. Mario unclogged Bowser's toilet and he flushed it, showing it was a success. This resulted in Mario getting rich, but he and Luigi only used the money for 'lotsa Spahgetti and Toast. Nobody cares, however.
Cardboard Mario: The Clogged Warp Pipe
The Plunger first appeared in the shame Cardboard Mario: The Clogged Warp Pipe, where it was needed to make it possible to go through the mysterious Warp Pipe that the game's about. Here, it is revealed that Mario didn't need the Cardboard Stars; He only needed a Plunger. This leads into another battle with Professor Frankly, who is revealed to auctually be some random hobo who had stolen some guy's house.