Entropy is one of those scientific principles that, supposedly, even a child can understand. In a simple analogy, it means everything becomes disorganized. Any child who has ever had to spend precious hours on a Saturday morning cleaning his or her room knows that it doesn't matter how clean the room gets, it will never stay that way. The chore will have to be done again. Entropy.
From a practical point of view, entropy means that useful energy dissipates and becomes useless. Take heat as an example. Let's suppose you have a hot cup of coffee. Just as you sit down to enjoy it, your neighbor knocks on your door and asks for help pushing a snow-bound car out of the ditch in front of your house. You bundle up, lend some muscle, and eventually come back to your coffee. It isn't hot any more. It's cold. Where did the heat go?
The heat leaked out of the coffee and into the room. It still exists, but it is spread so thin and over such a large area—the entire space of your kitchen, if not your whole house—that it isn't good for anything. And, you can't simply order it back into the cup. If you want the coffee to get hot again, you have to apply more energy. That's why they invented microwave ovens.
Couples who have been in romantic relationships understand this principle as well. At the beginning, they invest a lot of energy into each other and the relationship heats up. As time moves on and other things encroach into their lives, energy disperses. Then one morning they wake up and discover that the sizzle is gone. Entropy.
Faith also works this way. Pursuing it and maintaining it takes energy. Perhaps this is why Jesus commended the faith of children. When he suggested that we needed faith like that of a child, maybe he didn't mean simplistic, unquestioning faith, but rather that we needed a faith that hadn't been allowed to dissipate and become useless.