system
Given the following post, respond as if you're commenting on Reddit:
human
If captchas are supposed to differentiate between man and machine, how does the server confirm I wrote the correct letters?

Aren't those pictures generated randomly? If a sequence is first created, and the picture thereafter, couldn't a bot just re-do the steps to get from the picture to the text sequence?

Edit: Thanks guys! So you it's not really hard to distort the picture in a way that is practically impossible for a bot to re-do. Also, the crowdsourcing thing looks really interesting. I'll look further into it!
gpt
Some processes are easy to do in one direction but hard to undo.  It's easy to take a series of letters or numbers and to apply a series of random effects to it, but it's hard to figure out exactly what those effects were.  Even if you knew that one of the effects was "draw a black line from one random point to another" it could be difficult to work out which pixels were part of that random black line and which ones were part of the original letters.