Each week , we explain where American English expressions come from and how to use them . Today , we turn once again to the animal world . We talk about an animal that has been a great service to humanity : the Guinea pig . Guinea pigs are native to South America . Some people say the small , very social creatures got their name from the fact that they make noises like a pig . Another reason , they say , is that guinea pigs first left their homeland on ships that sailed between South America , Guinea and Britain . Other experts suggest that guinea pigs look like the young of another animal : the Guinea hog , also called the "river pig .” Over the years , guinea pigs have earned our respect . They have , after all , played an important part in keeping people healthy . For two centuries , they have been used for medical research . Their natural defenses against disease are similar to that of human beings . Scientists used guinea pigs in early studies of infectious diseases , such as tuberculosis and diphtheria . Today they are often used in studies of sexually transmitted and other infectious diseases . Rats are also used for laboratory experiments . But , as it turns out , Guinea pigs have more in common with people than rats do . So they are a better choice for some studies . These experiments have led to the use of “guinea pig ” in the English language . Some say that the term guinea pig , meaning “one subjected to an experiment ,” was first used in the 1920s . And this expression is still very common today . If someone calls me a “guinea pig ,” it means that new ideas or methods are tested on me . I am part of some kind of experiment and it does not have to be one in a laboratory . Let's hear this expression used in some examples . I have some new dishes to serve at my big dinner party ! But I've never made them before . What if they don't taste good ? How about if I come over this weekend and you can cook them for me ? You can be my guinea pig ! You can call me whatever you want . As long as I get to eat your great cooking ! This milk smells funny . Will you try it and see if it's old ? I'm not your guinea pig . Try it yourself ! The city government is trying out a new recycling program . It is the first of its kind in the nation . Why must our city be a guinea pig for every ‘great ’ idea our politicians hear about ? And yes , I said ‘great ’ with quotes because they're not great at all ! I heard your quotation marks . The expression , “guinea pig ” carries a lot of meaning with only two words . Calling yourself a “test subject ,” sounds way too scientific . And we just do not use it that way . A similar term , “lab rat ,” is rarely used and it does not sound very nice . And saying that you “are being experimented on to see if something works ” is so very…long . Guinea pig is much more common and you can use it in any situation . They may not like it . Another person who may not like this expression is someone who takes care of one as a pet . These animals do much more than help in scientific research . In the United States , many people own one or more guinea pigs , much like you would a dog or cat !