Parents despairing over whether their baby is hungry, tired or needs changing should look into their eyes to find out, scientists claim.
By listening to the pattern of the infant's wails and checking whether their eyes are open or closed, parents can determine the reason for their distress.
Babies which are angry or afraid will keep their eyes open, but when a young child is in pain they will close their eyes, according to a new study.
Changes in the dynamics of the cries can also be revealing, because they will become less intense if the child is angry but more intense if pain or fear is the cause.
The researchers, from the University of Valencia, studied 20 babies aged three to 18 months and studied them while they cried, as well as observing how their parents responded.
Pain was the easiest emotion for mothers and fathers to diagnose, but anger and fear were much harder to interpret, according to the study published in the Spanish Journal of Psychology.
Dr Mariano Choliz, who led the study, said: "Crying is a baby's principal means of communicating its negative emotions and in the majority of cases the only way they have to express them.
"Although the observers cannot recognise the cause properly, when babies cry because they are in pain, this causes a more intense affective reaction than when they cry because of anger or fear.
"When babies cry because of anger or fear, they keep their eyes open but keep them closed when crying in pain."
Nick Collins Telegraph.co.uk