I think calling this a Captcha isn't a greay way to explain what this is about. Bots spamming your sign up page are playing an economics game. Can I quickly an

I think calling this a Captcha isn't a greay way to explain what this is about. Bots spamming your sign up page are playing an economics game. Can I quickly and cheaply enough do whatever this bot action is so that the end result is worth the cost? Captchas were good because you need humans to verify them, which raised the cost to around $0.05 per action. But now ML breaks most captchas cheaply and easily. Proof of work based 'captchas' are another solution. They slow down the bots, and cost compute time, reducing the economic efficiency of the attacker. They make it not worth the effort. And if the attacks continue, you can autoscale the difficulty. And for humans, it's a minor slowdown without requiring any mental effort from them. It's a nice solution.