Sneeze Hold Your Breath Blink Swallow 
Answer: Hold Your Breath
If you overhear a bunch of kids arguing over how dangerous exposure to space is without a space suit or craft, it sounds like a horror movie. You’ll hear descriptions of blood boiling, instantly freezing to death, and other terrible ways to perish as your ship drifts away after your accidental airlock ejection.
These popular myths are just that, however, and humans can actually survive unprotected exposure to space for short periods of time without any sort of horrible and instant injuries. If you were to find yourself unfortunately outside a spacecraft or in the airlock with a loss of pressure (and no spacesuit to speak of), you wouldn’t have to worry about your blood instantly boiling.
In fact the biggest risk to humans in a vacuum is reflexive breath holding. You won’t instantly freeze to death in space (the vacuum slows the transfer of heat pretty significantly) and the sun won’t burn you up (although you may experience sunburn if you’re on the sun-facing side of your ship), but what will do serious damage to your body is holding your breath. The pressure differential between your packed lungs and the emptiness of space can cause permanent damage and even death as the gases in your lungs expand (but even then you won’t explode despite popular opinion on the matter).
Although such a scenario, floating totally unprotected in the vacuum of space, is improbable for most people (even astronauts), the research on the subject and the knowledge gained from animal studies and case studies of the rare times humans have come in contact with such conditions are critical for preparing astronauts to deal with more mundane but dangerous situations like leaks and failures in their space suits during space walks.
What should you do if you’re an astronaut with a leaking suit? Don’t panic, and don’t hold your breath. You’ll lose consciousness very quickly, but studies on the matter indicate that as long as your quick thinking astronaut buddies get you back into a pressurized environment within a few minutes, your body will re-pressurize and you’ll be fine.
Image courtesy of NASA.