>To avoid this, there needs to be a way to generate them cheaply, so you pretty much need a computer that can solve the problem in the first place. I don't see

>To avoid this, there needs to be a way to generate them cheaply, so you pretty much need a computer that can solve the problem in the first place. I don't see how that follows. Many captchas are just text with a confusing grid overlayed - that's incredibly trivial to produce, and producing one tells you next to nothing about how to solve them. Or if that's too easy, throw in some random distortion - still easy to produce, even harder to read.