" A “rich environment” is a combination platter of novelty, unpredictability, and complexity — three elements that catch and hold our attention much like risk. Novelty means both danger and opportunity. To our forbearers, a strange scent in the wind could be prey or predator, but either way it paid to pay attention. Unpredictability means we don’t know what happens next, thus we pay extra attention to what happens next. Complexity, when there’s lots of salient information coming at us at once, does more of the same. "

― Steven Kotler , The Rise of Superman: Decoding the Science of Ultimate Human Performance