True story: in UK schools they used to run an extra-credit annual maths test called the Maths Challenge (or something equally imaginative).
The test was a series of 20 maths problems - "a snail climbs up from the bottom of a 10 ft well, moving up five inches every day and slipping back three inches every night. How many days does it take him to escape from the well?" being typical.
You started off with 20 points, scored +5 points for a correct answer, -1 points for a wrong answer and 0 for a question you didn't attempt (so your final score was in the range 0-120).
Pretty much all the kids worked out immediately that this meant you could answer 4 questions you weren't sure of for every question you were, and you'd still end up with at least a few additional points... all except my friend Dan.
Dan ploughed ahead, attempting nearly every question on the paper. When he got his mark back he'd only scored around 15-17. He wasn't too bothered, however, until we realised that he'd scored worse than the 20 points he'd have got if he hadn't answered a single question.
In other words, if the chair he was sat on had taken the test, it would have scored higher than he did.
This is the true story of how my mate Dan was once proven to be dumber than furniture.
(originally posted on reddit)