Digital computers and human brains have different strengths and weaknesses. If you give me a long list of arthimetic problems to solve, at some point I’ll slip up. “How did I get the wrong answer on that division problem? Oh yeah, I thought there was a 3 in the tens place of the dividend when it’s actually a 6. I’ve just been staring at all of these numbers for so long that I’m starting to get tired!” But CAPTCHAs are easy for me to use, and they somehow impede malicious web bots that try to break through authentication barriers.