eli5: How do Captcha’s know the correct answer to things and beyond verification what are their purpose?

Question

I have heard that they are used to train AI and self driving cars and what not, but if thats the case how do they know the right answers to things. IF they need to train AI to know what a traffic light is, how do they know im actually selecting traffic lights? and could we just collectively agree to only select the top right square over and over and would their systems eventually start to believe it that this was the right answer? Sorry this is a lot of questions

In: 3352

Answer 1

With the 2 word ones that used to be more common before the select a picture ones, there was one easy to read word and one difficult to read word. The easy one was a the control word, so you could just answer that one correctly and put whatever you wanted for the difficult one.