In thinking about the challenge of getting artificial intelligence (AI) to understand our complex world, I recalled a Twitter post from a user of Tesla’s self-driving system. The user tweeted that his car kept stopping abruptly at a particular location for no apparent reason. Then he noticed a billboard advertisement on the side of the road, featuring a sheriff holding up a stop sign. The car’s vision system had interpreted this as an actual stop sign, and slammed on the brakes.