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 his thumb. In fact, she could thumb her nose at him. Thumbing your nose at someone is showing a person disrespect and disregard. This expression can also be used for things that you don’t respect, for example a type of literature or art or music, or even a law.

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. But if the crowd gave the thumbs down, he would be killed.

These days a thumbs up or thumbs down vote can be important but not so … violent.