Imagine it is a beautiful spring day in the month of May or early June.

You are sitting among a crowd of people at a large theater or stadium. Around you are hundreds of young people. They are wearing long robes over normal clothing and hats in the shape of a square.

Maybe some of your friends or family members are sitting behind you. Then imagine you turn around to look at them. Some are happy, while others start to cry.

Why? You just completed a two- or four-year degree program at a college or university, and are now attending a graduation ceremony. This event, also called commencement, is where schools recognize the students who successfully completed their degree programs.

Suddenly, a person you may have seen on television stands up, walks in front of the crowd, and starts to say something like this:

"Graduates, faculty, parents ... and old people that just come to these things, good morning and congratulations to the Dartmouth class of 2011."

They are giving a speech that marks how important this special event is to the graduating students and their loved ones.

Dilip Abayasekara is a former president of an organization called Toastmasters International. Toastmasters operates in about 145 different countries. It helps to train people in public speaking.

Abayasekara says speeches have been a big part of graduations in the United States and other countries for about 100 years or more. He adds that colleges and universities carefully choose who will give a commencement speech. Schools often choose people who are famous or successful in fields like politics, business or the performing arts. They do so in order to make the ceremony a final lesson for the students in addition to a celebration.

Abayasekara spoke to VOA Learning English.

"Perhaps the public at large will get a glimpse into the forces that shaped this person. And they will also get a glimpse into what kind of message this person wants to leave in the hearts of the next generation of leaders."

Many famous people have given speeches at graduation ceremonies over the years. In 2016, some of the most well-known speakers were film director Steven Spielberg and businessman Peter Theil.

Most commencement speakers offer advice, such as believing in oneself or never giving up when life becomes difficult. But what makes a speech truly great? What are the qualities in them that graduates will remember for the rest of their lives?

John Gabrieli is a professor of neuroscience at the Massachusetts Institute of Technology, or MIT, in Cambridge, Massachusetts. He studies memory. Gabrieli says memories are created from events when a person experiences a strong emotional reaction. Graduation ceremonies often fill people with many emotions, including happiness and feelings of success.

Yet while Gabrieli has heard many commencement speeches in his life, he admits he cannot remember any of them that well. Many speeches are similar, often just celebrating the students and thanking the people who have supported them. This does not make for strong memories, he says.

"What we remember, besides emotion, is what is unexpected. So daily expected things we forget very quickly ... So a commencement speaker has a challenge, if it's going to be memorable, to say something unexpected."

Gabrieli notes truly memorable commencement speakers either change the way the speech is given or make challenging, unpredictable claims.

Take Michelle Obama, for example. She spoke at a graduation ceremony at the City College of New York in 2016, while her husband was still president. She told the graduates that every day she "wake up in a house that was built by slaves." She was talking about the White House.

Here is another example. Actress Rita Moreno spoke at the Berklee College of Music in Boston, Massachusetts. During her speech to the graduates, she began speaking this way.

"I agreed to speak at this commencement and chose to bring the instrument. My choice: a voice, to articulate, to lay it straight. For on this date, my fate is to relate with, rhythm and passion, that you have a mission."

However, MIT's John Gabrieli warns speakers their job is to honor and prepare the students as they leave the school. An unexpected speech may make the attendees unhappy.

Geoffrey Cowan adds there are other ways a commencement speech can fail. Cowan is a professor of communications at the University of Southern California in Los Angeles. He was also once the director of VOA. He says speakers often forget public speaking is all about building a connection between them and the people in the audience.

"The worst thing to do is to talk down to your audience, and there's often a tendency to do that, or to talk in a mechanical way. And the other thing that's terrible ... is when people lose their place in a speech, and it becomes incoherent."

Cowan thinks the best commencement speeches center the audience's attention on a single idea. Often this can come from a personal story from the speaker's own life. For example, businessman Steve Jobs told Stanford University graduates in 2005 about how he left college without completing a degree program. Jobs also spoke about learning he was dying of cancer and how that changed his ideas about the world.

Cowan says the best speeches also present college graduates with a challenge. For example, in 2008 writer J.K. Rowling asked graduates at Harvard University not to be afraid of failure, but to learn from it. And in 2005, writer David Foster Wallace asked graduates at Kenyon College to try and understand and value other people's opinions.

If a commencement speech is truly great and memorable, it will not only serve to inform and motivate the graduates, says Cristina Negrut. Negrut operates a website called Graduation Wisdom. Since 2006, she has used her website to gather the best commencement speeches as well as advice on how to write them. She says that before the internet, many of these speeches could easily disappear with the graduate's memories.

"Now, with social media, the speaker ... ends up giving the speech not just to the university and the audience they have in the stadium, but to the world."

Negrut says these speeches can be an important source of knowledge. That is why the best ones are recorded and shared long after the ceremonies have ended.

I'm Pete Musto.