It's been a long day of roaming the most Eevee-heavy areas of your city to gather Eevee candy, but at long last, you've got 25 of them. A quick check of the weather report confirms that the storm you saw across the bridge will be hitting your town shortly. Your team is, of course, Instinct, and the Eevee you plan to evolve only knows Tackle and Tail Whip. All the conditions seem right to net you your favorite Pokemon, Jolteon. A flash lights up the night sky, and a boom only a second or so later tells you that the storm is right on top of you. You fire up Pokemon GO, select your Eevee, and pull the trigger. After seconds of tense spinning and flashing, you eagerly greet your new... Vaporeon?! As it turns out, there was a very simple way you could have gotten the Eeveelution you wanted.
Fans of the original Pokemon anime, the first season with Misty, Brock and the three Pokemon GO starters all accompanying Ash and Pikachu, will recognize the names Rainer, Pyro and Sparky. Those are the Eevee brothers, a trio of trainers that Ash and his group encounter. Sparky owns a Jolteon, Pyro owns a Flareon, and Rainer owns a Vaporeon. It would seem somebody at Niantic is a true blue Pokemon fan, because those names are how you choose which Eeveelution you want.
It's quite simple, really; all you do is name your Eevee one of those three names before evolving it. Since there are no fire stones, thunder stones or water stones in Pokemon GO, this is the only surefire way to control the evolution of the versatile Eevee. Naming your Eevee Pyro will net you a fire-spitting Flareon, naming it Rainer will get you a Vaporeon, and naming it Sparky will make friends jealous of your new Jolteon. Reports are coming in, however, that say that this only seems to work one time per name. Whether it could be used again if you traded away the Eeveelution is unclear, since trading isn't in the game yet and will be coming in a future update.