[hnphillipj]

never thought of this.

[linkingday]

But how does Captcha know that your answers are correct? There's likely already a predictive model that knows (within some confidence intervals) which ones are the 'correct' answers - someone had to label that data to begin with! Chicken, meet egg.

[joncrane]

They display the same pictures to multiple people and have some kind of agreement algorithm.