[jjcm]

You'd have extremely low entropy though. If I write an auto-signup script that just selects one or the other randomly, I'm still getting 50% of my botted accounts through.

Sure you can add more, but how many can you add before users get upset about doing the same thing over and over again? Three? Four? At four times I'm still looking at a 6% chance of getting an account created if I just select randomly. That's plenty.

Suggestions to use things that humans can easily distinguish but computers can't as captchas come up often. However, being only human readable isn't the only requirement for a captcha - another requirement is that it can't be computer guessable. If a computer can guess correctly with a high (>1%) chance of success, then the captcha is pointless.

[stinky613]

Oh I'm not advocating that this actually being used as a captcha. Though, as a thought experiment, I think it would be more effective if you had, say, eight gifs. Instead of an either-or choice you could ask the user to choose all that match a given condition.

I had a teacher who employed "multiple multiple choice" questions on his exams. The question would have some number of choices (e.g. a,b,c,d,e) and you had to choose the correct answers. These questions were tricky because the answer choices could all be wrong (meaning you select none of the answer choices), could all be correct, and any combination of the answers could be correct. It made guessing go from a 1 in 5 situation to 1 in 32. I was not a fan.