"Bees also display optimistic and pessimistic emotional states. In such tests, bees first learned that one stimulus (such as the colour blue) is linked to a sugar reward, while another (such as green) is not. They were then faced with an intermediate stimulus (in this case, turquoise). Intriguingly, they responded to this ambiguous stimulus in a 'glass half full', optimistic manner, if they had encountered a surprise reward (a tiny droplet of sucrose solution) on the way to the experiment. But if they had to suffer through an unexpected, adverse stimulus, they responded in a 'glass half empty' (pessimistic) manner."