Peter Rowlett introduces seven little-known tales illustrating that theoretical work may lead to practical applications, but it can't be forced and it can take centuries. As a child, I read a joke about someone who invented the electric plug and had to wait for the invention of a socket to put it in. Who would invent something so useful without knowing what purpose it would serve? Mathematics often displays this astonishing quality. Trying to solve real-world problems, researchers often discover...