This is what I do on my own sites too. Of course, given some bots relay the captchas to humans, this arguably works best for a site with a specific niche in mi

This is what I do on my own sites too. Of course, given some bots relay the captchas to humans, this arguably works best for a site with a specific niche in mind, since they can ask a significantly more difficult question that only someone interested in the topic would know. Like say, how a PhD maths related community or blog might ask a university level maths question, a chess forum may ask a chess related one and one about a certain gaming franchise might ask questions about said franchise. Bonus points for keeping out people with no genuine interest in the topic.