Your only stupid when you do stupid things.

My interpretation: Stupid people do stupid things.

It means that and intelligent person who does stupid things is still stupid. You are what you do. Another variation is "beauty is as beauty does" or "ugly is as ugly does". http://www.phrases.org.uk/bulletin_board/19/messages/1045.html It means actions are more important than appearances. Originally "handsome is as handsome does," and a number of other variations. Don't judge "stupid" until you see stupid in action. http://www.phrases.org.uk/bulletin_board/55/messages/411.html HANDSOME (PRETTY) IS AS HANDSOME (PRETTY) DOES - Good deeds are more important than good looks. The proverb was first recorded by Chaucer in 'The Wife of Bath's Tale' (c. 1387). In 1766, in the preface to 'The Vicar of Wakefield,' Oliver Goldsmith wrote: 'Handsome is that handsome does.' First attested in the United States in 'Journal of a Lady of Quality' (1774). The saying is found in varying forms, including 'Beauty is as beauty does'." From the "Random House Dictionary of Popular Proverbs and Sayings" by Gregory Y. Titelman (Random House, New York, 1996). Then there's the line from the movie "Forrest Gump": "Stupid is as stupid does." http://www.phrases.org.uk/bulletin_board/14/messages/118.html http://www.imdb.com/title/tt0109830/quotes

No rhyme or reason to foolishness or to fools practicing it.

It's expected for someone who you think is stupid to do stupid people to do stupid things.

If it looks like a duck and walks like a duck, it must be a duck.

It means if you say stupid things as a rule You are probably stupid.

Nothing of what Gump actually DID do was stupid (on the contrary!), so therefore he WASN'T stupid.