There is something almost irresistible about discovering an echo. Walk into an empty tunnel, stand beneath a great dome, or find yourself facing a distant mountain wall, and sooner or later someone will call out. Often, it is nothing meaningful—just a loud “Hello!” or someone calling their own name into the distance. Then everyone waits. …
There is something strangely passive about sleep. For roughly a third of our lives, we lie still, largely unaware of the world around us, unable to work, eat, travel, protect ourselves, or do any of the things that seem useful for survival. From an evolutionary perspective, this should make sleep expensive. An unconscious animal is …