It seems creationists never get tired of this one. I've lost count of how many times I've been told that it makes evolution impossible, and I just ran into someone using it again over here.
Let's begin by getting something straight. This is the actual, real life, second law of thermodynamics:
It is not some kind of cosmic legislation written down in the Universe's list of statutes that states "things shall not get more complex naturally!" It's a mathematical expression. More precisely, it's a mathematical expression that describes a specific set of conditions. But when you need to explain what that means to people who don't "speak math" you have to translate it into english instead, which is where the problem happens.
You can do it well:
"The total energy available to do work in an isolated system can never increase." or... "The total entropy of an isolated system can never decrease" or several other formulations that are all equally valid. (The "isolated system" part is particularly critical)
Or you can do it like creationists:
"The Second Law of Thermodynamics, simply stated, says that everything has a tendency to run down, decay and get worse, if left to itself.
Click the link, I don't make this stuff up. They actually say this. The second law of thermodynamics makes abandoned houses get run down and cars left outside get trashed. It's not just that guy, I've heard this so many times I've lost track. Ugh.
While we're on the subject, you can do the same thing with the meaning of entropy.
Real definition: Entropy = The total energy in a system no longer available to perform work.
Creationist definition: Entropy = Disorder and decay!
Now to get into why creationists have no idea what they're talking about when it comes to thermodynamics we should start by defining some terms...
Entropy: See above
System Boundary: The dividing line between the system you are examining and it's surrounding environment. Can be physical, or simply definitional... but MUST BE SPECIFIED.
Open System: A system which can exchange matter and energy with it's surroundings across the system boundary.
Closed System: A system that can exchange energy but NOT matter across it's boundary.
Isolated System: A system that cannot exchange either matter or energy across it's boundary.
It's only in that last one where the second law says you can't see a decrease in entropy. Where there is no external source of energy providing fuel for the process the system is going to continuously use up the energy it has available to work with until it's all gone.
Creationists try to claim the second law means evolution is impossible because it causes localized decreases in entropy. For the second law to actually say that was impossible evolution would have to be doing this without getting the energy for it from anywhere... the earth would have to be an isolated system that does not receive any external energy input.
So... the next time a creationist tells you the Second Law makes evolution impossible then assuming it's daytime tell them to go outside, look up, and ask them if they see a giant ball of burning plasma in the sky bombarding them with light and heat. (a.k.a.... energy!)
I almost feel embarassed to be burning a post on such a worn out issue, but as long as creationists keep using it people need to keep correcting them I guess.