Vault Boy poses with his hand stuck out in front him, thumb pointing up, and a wink and a smile. Most people probably think this is just him being a positive, chill, corporate bro, but there’s plenty of reason to believe that Vault Boy is doing something else entirely: squinting at a mushroom cloud. The reason people think this — and plenty do — is that Americans used to be taught that if a nuclear bomb exploded in the distance they should hold out their arms, stick up their thumbs, and see if the cloud was bigger or smaller than their opposable digit. If the cloud was bigger than your thumb, teachers explained, you’d know that you were in the radiation zone and should start running.