“In the last 15 years, the science of mind wandering has mushroomed as a topic of scholarly study, thanks in part to advances in brain imaging. But for a long time, it was still difficult to see what people’s brains were doing outside the lab. Then, when smartphones came on the scene in the late 2000s, researchers came up with an ingenious approach to understanding just how often the human brain wanders in the wilds of modern life.”