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Sometimes，you just can’t help it．Maybe you’re watching a sad movie，or thinking about the friend who moved away．Next thing you know，your eyes are watering，and you have tears running down your cheeks．Why do people cry when they are emotional? What are tears? Scientists are working hard to find the answers to these questions．

Ad Vingerhoets is a professor of psychology at Tilburg University，in the Netherlands．He is one of the few scientists in the world who have studied crying．According to Vingerhoets，there are three types of tears．Basal tears are the first type．They lubricate(润滑)the eyes and act as a protective barrier between the eye and the rest of the World．Next are reflex tears．They wash your eyes clean when something gets in them．Finally，there are emotional tears.“These are released in response to emotional states，”explains Vingerhoets．“Especially when we feel helpless．”

Scientists believe that crying has something to do with how humans developed and learned to depend on each other．“Humans are very complex social creatures，”says Lauren Bylsma，a professor at the University of Pittsburgh，in Pennsylvania．“It seems that tears serve to arouse help and support．from others，’’She says．“Another reason we weep is that humans have the longest developmental period of almost any animal．It takes a long time to grow up．”

Vingerhoets agrees．“I think that the reason why humans shed tears(流眼泪)has something to do with our childhood，”he says．“That’s the time when we are stir dependent on adults for love and protection and care．The major advantage of emotional tears is that you can target them at a specific person．”Vingerhoets says this ability to target someone could have come in hand in prehistoric times，when humans were living among dangerous animals．Crying could attract predators(捕食者)．Tears were a safer way to get attention．In this case，it is better to use a silent signal to ask for help，”he says．

Vingerhoets and Bylsma do frequent studies to better understand why humans cry．According to Byhma，there is still much more to discover．“It’s surprising，”she says，“how much we stiil don’t know．”