In 1872, Charles Darwin pub­lished The Expres­sion of the Emo­tions in Man and Ani­mals, a book that cat­a­loged emo­tional expres­sions in humans and their link to the animal world. In the … read more »

In 1736 the Swiss math­e­mati­cian Leon­hard Euler ended a debate among the cit­i­zens of Konigs­berg, Prussia, by drawing a graph. The Pregel River divided the city, now Kalin­ingrad, Russia, into … read more »