[ganafagol]

This is the best comment, by far. Of course we can have lots of brainstorming about other fun captcha-ish constructions, but the key question is whether they satisfy their purpose, which is to filter machines out and only those.

In low scale settings, if it's a place almost nobody knows, then this works, but many approaches work like just asking "What is 1+1?". Scale is low, no bot writer will bother to adjust.

In high scale, high visibility, none of this works. The incentive to break your captcha is so high that you'd need to basically construct a reverse turing test. You need to assume that the attacker is very powerful and very smart and will spend months custom tuning their solution to vreak your captcha. This is really hard and how to do it is the interesting discussion.

In summay, the setting matters. If it's the first one, we can debate toys all day long but they only have entertainment value. If it's the second then this is really hard and state of the art is to click pictures with traffic lights.