This is most obvious when we consider the limited resource that is our time. We have a limited amount of time that cannot be returned or repurposed once it is used up. There are no second hand shops to pick up used time and no way to magically grant yourself more time (in fact, the irony stands that most of our efforts to create more time in our lives, whether by exercise, changed eating habits, etc, are only nominally helpful in creating more time in our lives -- for there is actually time lost in accomplishing these tasks).
Now here is the grand question of opportunity cost that you need to ask yourself with every task.
What did I give up for this?
Because you did give something up. The choice to watch television, go for a walk, take a nap, hike a mountain, bike a trail, or write a book all came at the expense of something else. And our ability to multitask can only minimize that opportunity cost a small amount (and a lot of research suggests that humans are very poor multitaskers, as the final products of two tasks done at once are far worse than the same two products done in sequence).
What did you give up for that hour of video gaming or television watching?
What could you have done instead of browsing social media?
What has gone undone because you are reading this post?
I'm not suggesting that television and social media are bad and I'm DEFINITELY NOT suggesting that reading this blog is bad. In fact, I think we should be assessing everything that takes our time, good or bad.
Our hours our limited. The race is short. And I, for one, intend to do all I can for eternal gain, not for fleeting, temporal pleasure and self-glory. I will stumble along the way (wikipedia is my personal time sink). But I don't want to look back over my life and see an opportunity cost that far outweighed the choices I made with this one life.