You have just unwrapped, unpeeled or somehow exposed an entrée right before lunchtime. You're half-starved and excited to scarf, and suddenly, without warning, your hand slips and you watch impotently as your morsel falls to the floor.
There's no one around. The floor looks clean. It's time to make a decision. "Five second rule," you cry, snatching the snack from the jaws of microscopic monsters.
Perhaps you blow on it before popping it in your mouth, perhaps you don't, but the question remains, "Is what you have just done safe?" What you've done is definitely gross, but safe? Maybe not.