It’s 3 a.m. in Nairobi, and Ken’s laptop and phone glow in the dark. On one screen, waves crash against a beach in a video, and on the other, a woman stretches into a yoga pose. He has watched each clip several times, trying to decide whether or not it’s in slow motion. His phone buzzes. In a WhatsApp message, a teammate informs him that she has already labelled 2,200 video clips that day, 200 short of her daily goal. “Are you close to the target? I am so sleepy, I think I will continue tomorrow...