(1) Setting in an Alternative World

Sci-fi stories are often set in amazing locations like outer space, or deep in the ocean, or on flying islands. These fantastic settings are a fixture of the genre.

(2) Allegory

Sci-fi works are often allegorical, which means that even though they're set in a distant or fantastic location they're actually talking about our very own world. In other words, sci-fi writers like to comment on our own society in a sneaky way.

(3) Non-human Characters

Hurrah for aliens! And not just aliens—in sci-fi we'll find thinking machines, semi-human creatures and all other kinds of weird living (or living-ish) beings. These non-human characters are as big part of sci-fi as crazy helmets and shiny pants. (Okay, they're actually a bigger part).

(4) Time Travel

Sci-fi has us zipping back and forth in time. Sometimes characters actually travel back and forth in time (using time machines, natch), but time travel also refers to the fact that much of science fiction is set in the future. A good sci-fi novel will often have us readers "traveling" to the future.

(5) Science and Technology

No big shocker here, right? Sci-fi works are full of science and technology. Sciences (real or imagined sciences) are a big theme in the literature, and we'll also find all kinds of cool gadgets. Spaceships and submarines and super-powerful computers, oh my!

(6) Journey

Considering that sci-fi is, at its core, all about adventure, is it any wonder that journeys are central to this literature? We won't find stories about a guy walking his dog around the park in this genre. It's more like a guy hopping on his spaceship and circling the universe.

(7) Dystopia

It's the opposite of utopia. Still confused? A utopia is a perfect society where everyone is happy. A dystopia is a society in which things seem good on the surface, but actually everyone is miserable inside. It's a society that's oppressive.

(8) Age of Reason

We're not talking about age 18, or age 21, or even 50. We're referring to the age—beginning in the 18th century in Europe—when there was an emphasis on the rational understanding of the world and exploration. During this period all of those superstitious beliefs of the Middle Ages were dumped out the window.

(9) Advances in Science and Technology

Beginning in the 16th century with the Scientific Revolution in Europe, big leaps were made in scientific knowledge. Technology also developed in leaps and bounds, especially in the 19th century. These advances in science and technology laid the groundwork for the development of early Science Fiction.

(10) Science Fiction Magazines

Sci-fi magazines, which were first published in the first half of the 20th century, played a big part in the development of the genre. Some of the most famous sci-fi works—like Isaac Asimov's Foundation—were first published in sci-fi magazines. These magazines brought sci-fi to the masses and helped popularize the genre.