There was this enormously smart guy called Karl Popper. Born around the time Einstein was having brainstorms, and dead some years after the fall of the Soviet Union, which must have given him immense personal satisfaction (he'd been a Jew in Austria in the 1930s, so he hated totalitarianism in all its shapes and sizes). He's a famous philosopher and has said some very interesting things in his career, but probably the most remarkable bit for the layperson is the concept of falsifiers. This is important because it is the current measure of a theory being scientific.
Here's the point in a nutshell. Anything we can objectively state about how the world works, is based on what we know about it and can logically assume from that. Then again, there are more things between Heaven and Earth, friend Horatio, yadda yadda. So every scientific theory can be disproven by showing that it is incorrect. This is important, so try to follow me here: if you can find a case where the scientific theory doesn't work, then the theory is proven to be wrong. No "ah, but", no "exception proving the rule", nothing. If you have a theory that has been tried out a lot of times, the theory is extremely likely, but no matter how well supported it is, if you can show it to be false, it's false, and the fully tenured research scientists before you are utter morons.
Let me make sure that you've understood this right. It's not like, "the apple has always fallen down from the tree, but there's a minuscule chance that next time it might just fall up". (No, that's quantum physics. Har har.) Rather it's, "the apple has always fallen down from the tree, and it will continue to fall down from the tree; but if somebody shows me a tree, without magnets or anything, where the apples fall up, then I'm gonna shut my big mouth and go teach high school science".
Now, here's the fundamental bit. Because of what was said above, a scientific theory is only valid if it allows the possibility of an event that would disprove it. You could say, "Apples fall down from trees, and that's because all objects are attracted to one another with a force directly proportional to their mass and inversely proportional to the square root of the distance between them". It would be a scientific theory - because eventually somebody could come along and prove that apples fall down from trees because there are metals in the grass and metals in apple juice which form a strong magnetic force. It's implausible, but it could happen - and that's why the theory is scientific.
The event that could conceivably disprove a theory is called a falsifier. Now, here is the entire point of this enormous multi-part post:
If a theory has no falsifiers, it is not scientific, and therefore it is wrong. I don't have to prove that your theory is wrong. If your theory doesn't include a situation where you would say, "Oops, looks like I'm full of shit", then you're full of shit.
Join me in Part III, where I discuss why creationists are morons.