Imagine, for a moment, you have a cup that holds eight ounces of water. If you pour four ounces of water into it, is it half full, or half-empty? The answer to that question is one we use to describe people's outlook on life and the world around them.

Some people look at an eight-ounce cup with four ounces of water as being half full. We call them optimists. They see four ounces of water as more than nothing and that's a good thing. On the other hand, some people see it as half empty. We usually call them pessimists. 

We call them that because we assume that a full cup is an ideal situation, and anyone who sees the cup as less than that must have a negative mindset. What if, instead, we think of a different way to measure? What if we think differently about a half-empty cup.

Here's why it matters: A lot of us are walking around trying to fit 10 ounces of life into our eight-ounce cups--metaphorically speaking--and we're mostly just making a mess. We end up spilling on those around us because we can't possibly fit everything in our cup. At some point, it isn't complicated, it's just physics. 

There's a law of physics that says that two objects cannot occupy the same space at the same time. If you have a cup full of water, you can't add anything to it without displacing the water. If you drop in a rock, it will splash water out. If you pour in more water, it will run down the sides. 

Practically speaking, if your cup is completely full, you have zero margin and that's a bad thing. Margin is the difference between your capability and your responsibility. Or, said another way, it's the difference between the time and abilities, and the way you choose to spend them. If you have no margin, it means you have nothing left for anything else.

That's what is so meaningful about the "cup-half-empty" rule. Having room in your cup means you have margin. Margin is breathing room. It gives you space to think and focus. It gives you the ability to be intentional about what you are doing. It even gives you a chance to do more--to do something new.

Instead, most of us live with our cups overflowing. That, despite the biblical reference, isn't a good thing. At least, it isn't when it comes to having time and resources to do the things we want to do. It means that we're already trying to do more than we're capable of and that leads to burnout.

The goal, then, is pretty simple. If you want to eliminate stress and be more productive at the things that are really important, you need to create margin in your life. You need to have some room so that you can say yes when something comes along that would add value to your life. That means you need to get rid of some of what's already in your cup. 

That sounds easy, but the reality is, everything that's already in the cup got there for a reason. It's not as simple as saying no to the thing, you're also saying no to the reason it was there in the first place. Even if it's not easy, however, it's still important. 

If you're a leader, this matters because one of the most important things you do is make decisions that affect the people around you. If you're completely tapped out, you have no mental space left to give careful thought to those decisions. You simply can't take anything else in, you're too busy trying to keep the cup from spilling.

You owe it to your team to create margin in your life. For that matter, you owe it to yourself. As a result, you'll make better decisions, which, in turn, will make you a far more effective leader.