Ideally, I'd prefer everyone to understand it, of course. Rather than keeping editing it, I'll post my latest paragraph here and, if you have time, I'd be interested to know whether you think this works better. If so, I'll edit when I'm back at the computer (heading out soon)

I then used the exercise to explain a bias called positive bias. First, I noted that only 21% of respondents reached the right answer to this scenario. Then I pointed out that the interesting point isn't this figure but rather why so few people reach the right answer. Specifically, people think to test positive, rather than negative, cases. In other words, they're more likely to test cases that their theory predicts will occur (in this case, those that get a yes answer) then cases that their theory predicts won't. So if someone's initial theory was that the rule was, "three numbers, each two higher than the previous one" then they might test "10, 12, 14" as this is a positive case for their theory. On the other hand, they probably wouldn't test "10, 14, 12" or "10, 13, 14" as these are negative cases for their prediction of the rule.

This demonstrates positive bias - the bias toward thinking to test positive, rather then negative, cases for their theory.