Pearls before Swine Bill Watterson Stephan Pastis collab #1 piece, of Calvin and Hobbes fame

In June 2014, three strips of Pearls Before Swine (published June 4, June 5, and June 6, 2014) featured guest illustrations by Watterson after mutual friend Nick Galifianakis connected him and cartoonist Stephan Pastis, who communicated via e-mail.[42] Pastis likened this unexpected collaboration to getting "a glimpse of Bigfoot".[43] "I thought maybe Stephan and I could do this goofy collaboration and then use the result to raise some money for Parkinson's research in honor of Richard Thompson. It seemed like a perfect convergence", Watterson told The Washington Post.[44] The day that Stephan Pastis returned to his own strip, he paid tribute to Watterson by alluding to the final strip of Calvin and Hobbes from December 31, 1995.
-Bill Watterson's Wiki