The strip mentioned in this page.
2-D Calvin is a period of time when Calvin imagines himself as being 2-D. 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 claims to his dad that he was two-dimensional all morning, which was why he didn't see him all morning, causing his dad to say "I'll bet you can't do it all afternoon" to which Calvin's Mom replies to this as Dear!
"It'll build character!"
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