Today we will be talking about a hated but misunderstood animal – the rat . Well , perhaps not everyone . In some countries , dishes made with rat meat can be rare and sometimes pricey -- what we call a delicacy . Plus , rats are useful . With their extreme sense of smell , people can train giant rats to find land mines and even tuberculosis . But do these things make people love rats ? For the most part , rats are not beloved animals . For starters , they're not cute . They have pointed noses and long , thin tails . They can eat and damage crops . And the world has long blamed rats for spreading diseases , like the Bubonic plague in Europe during the 14th century . But , perhaps we shouldn't be so quick to judge . Scientists now think that it was most likely not rats , but another rodent , the gerbil , that caused the Bubonic plague . They suspect that gerbils traveled to Europe from Asia , some along the Silk Road that traders used . But these animals were not carrying spices and silk , but rather disease . Today , however , gerbils are pets in many American homes . Teachers sometimes keep them in classrooms for students to care for . Rats , not so much . Such is the difficult life of an unwanted , misunderstood animal . So , scientists can debate the role of rats in spreading disease . But the fact that rats have a really bad reputation in American English is not debatable . It's the truth . None of our rat expressions means anything good . The simplest way we use this word is to simply say , “Rats !” Americans often use this expression when something goes wrong . The term is common and polite -- unlike some of our other expressions we might use when we are angry . As we said earlier , rats may have a good sense of smell . But smelling a rat isn't good . When we say , “I smell a rat !” we suspect that something is wrong . If you feel that someone has betrayed you , you can say that you smell a rat . A pack rat is not good , either . This is a person who keeps useless things . And worse , they live with all the stuff they have collected . So , calling someone a "rat " is never an expression of respect or affection . When describing people , a “rat ” is someone who is not loyal or cannot be trusted . A rat snitches on someone to an authority figure – a parent , a teacher , a police officer . As a verb , the word "rat " isn't good either . To rat on someone means to betray a loved one , friend or someone else you know . When you rat on someone , you tell on them . Let's say you know that your brother ate the last piece of cake when he wasn't supposed to . You rat on him to your parents . Or maybe you rat on a colleague at work . Ratting on people , or tattling on them , will not win you friends . It just makes you a rat . The words tattling and tattletales are often used for children . But ratting someone out or snitching on them can be for any age . No matter what your age , nobody likes to be called a rat , a snitch or a tattletale . However , it is a little different when the police are involved . Let's say you have information about a crime . When the police begin asking questions , you decide to keep that information to yourself . You may feel you don't want to rat on someone else . However , nobody would blame you for sharing information with the police if it helps them catch a criminal . Well , another criminal might not approve . Most criminals have a different code of conduct among themselves : You don't rat on fellow criminals to the police . In old police television shows and movies , you may hear one criminal criticize another who snitched to the police . They may say , "You dirty rat !" You would not say that a hardened , possibly violent criminal tattled on another ... unless you were trying to be funny . So , when using the word “rat ” in English know that the meaning is never a good one . But in life , maybe we should take another look at rats and give them a chance . Do rats have a good reputation in your country ? Please tell us ! It would be nice to know there is a place on this planet where saying "Rats !" is a good thing . You better watch out .