As Orson Scott Card says, "A rustic setting always suggests fantasy; to suggest science fiction, you need sheet metal and plastic. You need rivets." But if you're being scrupulous, it breaks down more like this: Magic gets to break the laws of nature. Science doesn't. And that goes for science-fiction, too: It might do things that aren't currently possible, but as soon as it starts breaking the laws of physics it has stepped out of science and into fantasy. Here are some of the red flags that will let you know you've trespassed into the heart of science fantasy turf.

The Precognition Myth
Precognition breaks one of the basic laws of physics, that of causality or the law of cause and effect. I know what you're thinking. In a film like Minority Report, knowing the future allows you to change it, thus preserving the laws of cause and effect. The problem here comes in when you look at how the information could come from the future to the present. To paraphrase physicist Michio Kaku, any electron from the future is just fulfilling its own past. In other words, it can't carry any information from the future without violating the laws of causality. Sorry, fans: Minority Report looks like science fiction, but it is pure fantasy. And that's nothing compared to...

Creature Creations
Everyone loves Godzilla, but let's be clear, he is physically impossible. Radiation giving rise to giant creatures has dominated movies ranging from Them! to Godzilla, and it is used the same way Harry Potter uses a wand -- as magic. But let's grant that bit of magic, because there are other giant creatures, like King Kong, who are just big naturally. Can they exist? Um. No. Do the math: When you double something in volume, its weight increases by a factor of eight. According to the biologist Michael Dexter, "Godzilla would weigh something like 60,000 tons." That's 60,000 tons of meat and bone. It would just pulp like jelly. Massive creatures would have problems with everything from support, to heat dissipation, to circulation, to their skin cracking and sloughing off. Not pretty. The only way around it? Helloooo, fantasy!

Inter-Species Mating
Poor Spock. Half-human and half-Vulcan, the Star Trek mainstay is doomed to fit into neither culture because he too strongly resembles the other. A lovely thought. Except, if humans are too different to cross-breed with our closest relatives -- apes -- then how, exactly, are we supposed to cross-breed with an alien species that has copper-based blood, of all things? It's...illogical, to say the least. The same goes for any alien-human hybrid. It's just not physically possible, unless you invoke, that's right, magic.

Ray Guns!
Everybody wants one, from Hans Solo to Captain Kirk. Granted, it's technically possible to have a ray gun. It's called a laser. There's just a few problems to overcome with the way they are depicted in film: First of all, lasers are invisible unless they are bouncing off something. So, if there's no dust in the air, you can't see them. Second, light doesn't stop until it hits something and is absorbed. Which means that firing a laser on a space ship is one of the dumbest ideas possible. Third, laser beams move at the speed of light. Dodgable? Not so much. And finally the largest problem is the sheer amount of energy it would take to vaporize someone. You'd need to connect the ray gun to a massive commercial power station. In other words, either Han Solo is dragging an extension cord through the Death Star or they're using magic. Oh, right! They ARE using magic. It's called the Force.

Now... Who wants to tell me why District 9 is a fantasy? Anyone?