I never thought it was improbable that Calvin could tie himself up. Knots are after all easier to tie than untie. Only the thoroughness of his restraints needs explaining by Wattersonian exaggeration.

I'd test it empirically by tying myself to a chair now, but I don't have an imaginary friend around to verify that I'm doing it properly.

What always bothered me more, for related reasons, was the strip in which Hobbes assented to Calvin's line 'I suppose it'll all make sense when we grow up'. Before then (and, really, after) I'd supposed Hobbes to be ageless, a kind of independent spiritual entity or morality finding a dependent physical manifestation. For Calvin to imagine him as his age made him too specific. But I mysticise.