“Write a gratitude letter to someone you’ve never properly thanked.”

This was the task I recently assigned as homework to my undergraduate class. I explained that the recipient of this letter could be anyone at all — a parent or older sibling, for example. Or they could choose a Little League coach, a rabbi, or a childhood friend — anyone who’d helped them along the path of life and to whom they owed a debt of appreciation.

I allotted students one week to complete this task and reflect, in their journals, on the experience.

As it turns out, students decided to thank a former teacher by a large margin — more than twice as often as they chose a parent, for example. Here’s what one student wrote:

I wrote my gratitude letter to my 12th grade English teacher, Mr. Daniel Reitz. He taught me Creative Writing, but more importantly, he was one of the first teachers who encouraged me to pursue what I loved. I attended a very competitive public high school, and it was difficult for me to understand what I wanted out of life besides being traditionally “successful.”

He encouraged me to be bold — at the time, it applied to my interest in music as well as to my graduation speech. He urged me to care more about those around me, too, with the care he displayed not just for me, but each individual student he ever taught. In addition, he kept up with me even after I graduated. The long after-school conversations I had with him, and his ability to empathize, changed my trajectory permanently, and I will always appreciate him for that.

Who benefits from gratitude letters? Research shows that the act of writing one reliably increases happiness. And the recipient likewise experiences a boost in happiness. How much? The writer expected their letter would make the person happy, but it turns out that the person they thanked benefited even more than they anticipated.

As with all assignments, I promised my students that I’d do the same work in parallel. And when I sat down to write my letter, I also ended up choosing a teacher.

Don’t assume that teachers can read minds. They may not know what a profound impact they have on the lives of their students.

Do encourage the young people in your life to write a gratitude letter. Now more than ever, it’s a good time to give thanks for the teachers who encourage us to pursue what we love, push us to be bold, urge us to care about the people around us, and change life trajectories for the better.

Angela Duckworth is cofounder and CEO of Character Lab and a psychology professor at the University of Pennsylvania. You can sign up to receive her Tip of the Week — actionable advice about the science of character — at characterlab.org.