There’s a video that made the rounds on Facebook about a little boy who comes home from school to find that his sister bought him a hamster with her own money.

After he takes a look at it, he gets down on the floor and starts to cry. And then his sister starts crying and there they are, two siblings entwined in one another’s arms, weeping with joy over a little rodent.

What makes this video so endearing are the things we don’t know. He might have had a bad day at school, or he might have had an argument with his best friend, but what’s certain is that this little gift will be a story he tells for the rest of his life.

The video made me think about how empowering it can be for other people when we give freely of ourselves. We don’t always know the impact we have when we donate money, or make an effort to visit someone who lives alone, or we save up all our money and buy someone a hamster. Giving changes things; it alters the course of the river.

There’s a delicate balance in the universe that operates just under our radar–one where the tiniest introduction of something new to a system irrevocably changes that system for eternity. One of the most powerful examples of this principle is found in a story that took place at Yellowstone National Park.

In 1995, rangers reintroduced wolves into the park’s environment. Upon hearing this, most of us imagine the destruction wolves can unleash. And while it’s true that wolves do kill certain species, what we might not know is that they also bring life to many others.

Wolves had been absent in the park’s environment for 70 years, which resulted in an over abundance of deer. The deer had reduced the vegetation to almost nothing. But with the introduction of a small population of wolves, the impact they had was beyond remarkable in thousands of perceptible and imperceptible ways.

First, of course, the wolves killed some of the deer, but that wasn’t the most important thing. The deer started avoiding certain areas of the park, especially the valleys and the gorges, and those places started to regenerate. Some of the trees grew four times in size so forests began to grow again, and the birds started moving in.

With more trees came more beavers, who created more niches in the water for reptiles and fish and otters and muskrats.

The wolves killed coyotes which helped regenerate the rabbit and rodent populations which meant more hawks and weasels and foxes and eagles. And then the bears moved in to feed on the carcasses the wolves left behind and to feast on the berries growing on the new trees.

But here’s where it gets really interesting: the wolves changed the behavior of the rivers.

The rivers began to meander less, were more directed with fewer channels and more pools. The regenerating forests had stabilized the banks and so the rivers became more fixed in their course and there was less erosion. So the wolves transformed not just the ecosystem of Yellowstone National Park, but also its physical geography.

The point is that we live in a connective universe, where the tiniest thing can impact and change our environment—and indeed our lives—in ways that we can’t even predict. I submit it is these small, unmentioned things that truly fuel the world.

Last week, a woman from a small New England town I once lived in died in a car accident. She left behind three sons and a daughter who wanted more than anything to hold a beautiful memorial service for her with flowers and a reception and a marker for the cemetery befitting of their love for her.

In 24 hours, they raised $25,000 and all of it came from the people of the town donating $20 or $50 or $10 to a crowd-funding site on the Internet. The sons were overwhelmed by the response, and you can be sure they will carry that generosity in their hearts and pay it forward for the rest of their lives. They will change the lives of a hundred people who will go on to change the lives of a thousand more.

And that’s how this all works, whether it’s a hamster or a wolf or a $20 gift. We can change the course of the mightiest rivers with the smallest of intentions. It’s such a powerful truth.