Before my bike-fatal road traffic accident, I had to park my wheels [synecdoche!] all over town. I needed a lock which was good enough to deter Bicycle Thieves but not so awkwardly heavy that I couldn't easily cycle with it on board. I found it also helped to deliberately 'distress' the frame with decals, mud spatter and and customising the handle-bars with sticky-tape. That and parking my bike next to a more obviously desirable bike (with crappier security). It was an optimisation problem.
Google acquired reCaptcha Inc in 2009 and threw a lot of software engineers at the problem and they were able to deduce humanity simply from the clicking style. I guess the 'bots click on the middle pixel of the tick-box while humans miss the bulls-eye and take longer to hit the outer rings as well.
Captcha got so distorted that although computers could figure them out 99.8% of the time, ordinary people were foxed 50% of the time and that meant lost business. How much do I want to access This Site? Enough to spend two minutes being told that I'm an idiot? Is there another place I can more easily do business? Megalithomania turned up to see The Ringstone, unannounced, on my birthday, a few years ago. I thought it would be nice to make contact but was so frustrated by the Captcha-portal
against the Blacks have low IQs finding questioned whether the dispossessed knew what the words meant when the IQ test was designed by Dr Strate Whyte-Mail PhD from Stanford: Which jacuzzi is the odd one out? A reCaptcha puzzle I was recently presented with asked me to count the number of shop-fronts rather than kittens. I could imagine a smart chap from Doha asking "what's a shop-front?". I recently labelled one of my graphs for 2nd Year ResMet course "Arbitrary Units" and two of the kids didn't know the word arbitrary in that context. I had similar probs with "salient" a few years ago.
And of course, it's war. If google and others are trying to separate humans from machines, curious researchers are trying to push the envelope to see if they can break in. Recent research published in Science and reported by George Dvorsky has developed a new way of looking at mangled letters and gets the right answer two times in three. I'm about ready to go back the parchment and quill pen.