Paul Rudd was really vying for the title of David Letterman's biggest fan on Tuesday's Late Show.
The 46-year-old actor revealed that he was once a member of the unofficial David Letterman fan club. "It had nothing to do with us," Letterman said, adding that the organization eventually got a cease and desist letter.
Rudd says he figured the 68-year-old comedian had nothing to do with it as the member gifts were underwhelming. "It was a terrible fan club," he recalled. "What it consisted of was a pen, a collapsible drinking cup, a sponge that if you put in water would grow ...back scratcher and a balloon."
Letterman -- who's only a few weeks out from leaving the CBS program-- said sarcastically, "And you're saying it was a lousy fan club?!"
While the Ant-Man star shared he wasn't a fan of the trinkets, he did reveal to the beloved host that he still had his button and untouched sponge after all these years. Letterman found it "heartbreaking" when Rudd began to pin his membership button to his suit.
The men then decide to finally dip the Letterman fan club sponge in water to see if it actually expands. "Three-fourths of my life later, we should see what happens," Rudd declared before sticking the sponge in a bowl of water.
"If you'd done this earlier, maybe you'd have a more positive view of the fan club," Letterman joked.
Paul Shaffer and the show's house band even got in on the experiment by playing some epic music while the sponge grew.
Thirty years later, Rudd pulled out the sponge and started wiping up Letterman's desk. "I never thought I'd clean your desk with it," he quipped.
For more Letterman hilarity, check him out in this special "Billy on the Street" segment: