Each week we explore the meaning and origin of American English expressions . Today , I want you to look at your hand . The hand is an amazing tool . With it , we humans have done many wonderful things . Of course , human hands have also done some not so wonderful things . But today , let's focus on the wonderful things . For those , we should thank the thumb . You could say the thumb made us humans what we are . Its development was as important an event in man's growth as his success in learning how to walk upright . Our opposable thumbs , as we call them , allow us to work with our fingers . Opposable thumbs also allow us to grab and pick up things . Scientists say without our opposable thumbs , humans might not have survived . The thumb is so important that we have a couple phrases to express its power . The first power thumb expression is the rule of thumb . These are not rules about how to use your thumb . A rule of thumb is a general principle developed through experiments rather than scientific study Another power thumb expression , is to be under someone's thumb which is not a good place to be . When a person is ruled by another , completely controlled by someone else , we say the person is under the other's thumb . For example , a sick woman may find herself under the thumb of a greedy relative . The relative wants to inherit the woman's money . He says he will take care of all her finances while she is ill . Soon , he does not buy her medicine or doctor appointments . The woman's health gets worse and worse , but she can't do anything about it . She is completely under his thumb . If this woman had control of her own money , she would not be under this man's thumb . In fact , she could thumb her nose at him . Thumbing your nose at someone is showing a person disrespect and disregard . No matter how you use your thumb , each hand has only one . And that's a good thing . Two or three thumbs on a hand might make doing things hard . In fact , that is how we describe people who are clumsy . We say they are all thumbs . Anyone can be all thumbs . You know the feeling . You're in the kitchen cooking dinner and you keep dropping things -- the spices , the broccoli , the knife . All go crashing to the floor . You might say , “What is wrong with me today ? I'm all thumbs !” So , it is a good thing that our hands are designed to have only one thumb . And if that thumb is green – even better ! Someone with a green thumb is really good at growing plants . Let's say the balcony of a neighbor's apartment has a beautiful flower and herb garden . Her roses and begonias are big and fragrant . Her parsley , chives and cilantro are so plentiful that she shares them with all her neighbors . As she gives a bunch of fresh herbs and flowers to you in the elevator one day , you say , “These came from your balcony garden ? Wow , you really have a green thumb . I can't get anything to grow on my balcony !” So , the thumb is helpful and useful . In fact , waving your thumb at a driver used to be a way of getting ride . Hitchhiking was common in the U.S . decades ago . But thumbing a ride , as hitchhiking is also called , is no longer considered a safe way to travel . In fact , it is even illegal in some U.S . states . However , giving someone a thumbs up is still legal and still common . Thumbs up means you approve something . Thumbs down means you don't . If a number of people are deciding on a yes or no question , someone can call for a thumbs up or thumbs down vote . This expression comes from ancient Rome when gladiators fought for their lives in the public arena . The crowd decided if the fighter was to live or die . A defeated gladiator would lower his arms as a sign of surrender . If the crowd turned up their thumbs he would be allowed to live . These days a thumbs up or thumbs down vote can be important but not so … violent . If someone asks you if you enjoyed this show , we hope you give us a thumbs up .