The strip mentioned in this page.
2-D Calvin refers to an incident in which Calvin imagines himself as being two-dimensional. He claims that he is "thinner than a sheet of paper...having width but no thickness..." To hide, he stands up straight and turns sideways so he is "a nearly invisible vertical line!" In the last panel of the strip, Calvin tells his dad that he was two-dimensional all morning, which was why he didn't see him. This causes his dad to quip, "I'll bet you can't do it all afternoon".