Back in college I learned about a half-life. It’s the time it takes for half of a substance to decay. The half-life of known radioactive elements range from 1.8 milliseconds for Unuoctium to 4 million years for Technetium.

But what’s the half-life of an act of encouragement? Just how long might a word of encouragement you give someone last?

Unlike the elements in the Periodic Table, an act of kindness has no a priori and fixed half-life. You just never know what will happen with the smile you give a stranger, or the kind words you say to a waiter, or the time you spend with someone in need.

Some acts of kindness, like Unucotium, decay rather quickly. Others last a lifetime.

I still remain grateful for Joseph Dionisio, a teacher in my high-school who took me aside as a awkward teen and invested in me. Despite my failing grade in Speech 101, he believed I could excel as a public speaker and he coached some early successes out of me. (Read the story here.) His investment in me will outlast my lifetime.

Others since have encouraged me too. In fact, I keep a special folder on my computer containing emails I’ve received from them. When I’m feeling down, I reread them to pick me up. The ones I treasure most are the serendipitous ones—those thanking me for something I had done for them without even knowing it. Reading those keep me on high-alert and remind me that at any time my words and actions can make a difference for someone.

I doubt Mr. D., as we called him then, thinks very often about the investment he made in me. But I do. I think about it every time I stand before an audience, for I likely would not be standing there if he hadn’t stood with me so many years earlier. And so, whatever good I might inspire in another, it seems to me, should be credited to his account—and, not only his, but so many others who’ve encouraged me along the way.

I often think of heaven as that time when we’ll learn the full extent that our words and actions had on others. We’ll be astonished at how we’ve impacted generations of lives we won’t ever know or even imagine—all because of because of something we’ve said or done for someone along the way. For now, I suppose, it’s a good thing that God keeps all of that a mystery. Most of us don’t handle well the little influence we already have. We just crave more. Ironically, that craving comes in the midst of so many whose lives could be changed by a warm smile or an understanding moment of our time.

I don’t know the half-life of an act of encouragement. The test for me is simply this: Do I leave people better than I found them? I’ll leave it for God to assign the half-life on those acts, just has He has for the rest of His creation.

What act of encouragement has meant a lot to you?