> Naive question: how can clicking on the motorbike or traffic light image help to train an ML algorithm if they already know what image has a motorbike in it, or otherwise the captcha would not make sense.
It's more than just your answers that are fed into ML and more than just what others have already said: there's also the way that your browser functions and the way you interact with it. Your IP address, browser, OS, screen size, input type, timezone and current time of day, how fast do you select different images, etc etc. All of this gets fed into ML algorithms and answers to the obvious images are used as corollaries to support/deny your ancillary information.
It's more than just your answers that are fed into ML and more than just what others have already said: there's also the way that your browser functions and the way you interact with it. Your IP address, browser, OS, screen size, input type, timezone and current time of day, how fast do you select different images, etc etc. All of this gets fed into ML algorithms and answers to the obvious images are used as corollaries to support/deny your ancillary information.