BeeDeeDubbleU - 11:43 am on May 22, 2012 (gmt 0)
The problem is that these questions, and particularly the obvious generic "color of coal" variants, will become machine-solvable in time. In much the same way that OCR has progressed in effectiveness. Unfortunately, anything your web visitor can do, a machine can do... it's just an arms race.
As I said earlier, I have been using this for years without a single auto submission. There is an infinite number of questions that can be asked and an infinite way of asking them. It would be one smart machine that could get past all of these. ;)