Take the 2-minute tour ×  ``Proofs without words'' is a popular
column in the Mathematics magazine.  Question: What would be a
nice way to characterize which assertions have such visual
proofs? What definitions would one need?  I suspect that in
order to make this question precise, one will have to define a
computational model for the ``visual verifier'' and postulate
the possibility of a visual proof if there is a quick
verification algorithm, and the visual proof itself is short.
Rev. 1:  To elaborate: the intriguing and essential feature of a
visual proof is that the proof can be ``read'' or verified
rather quickly using primitive operations that are different
than those involved in reading and verifying textual proofs
(e.g., area comparison or counting). These are operations that
are native and quick for the visual system. (There may be other
operations that could be used as well: color, texture, shape, or
any other visual cue could be part of the repertoire.) It is not
necessary that for an assertion to