Not quite a CAPTCHA, but something similar has actually been done before. The ESP game[1] paired two random people looking at the same image while a timer tick

Not quite a CAPTCHA, but something similar has actually been done before. The ESP game[1] paired two random people looking at the same image while a timer ticked down. The players had to enter labels that described the image, and if both players applied the same label, their score increased, and the label was associated more strongly with that image. Well-known labels for an image were excluded after a while, so you had to guess less and less obvious labels in order to score as time went by. Once in a while the system would also assign test images with known labels to prevent cheating. Apparently a lot of people played it because it was fun, so there wasn't even the need to pay them for labeling the dataset... [1]: https://en.wikipedia.org/wiki/ESP_game https://en.wikipedia.org/wiki/ESP_game