(This is based from the CYOTF (PG-13 Version) chapter)
What you don't know is the hot dog vendor is an evil scientist with an obsession with transformations, especially fictional ones. He has concocted a wicked plan that when one of his drink are drunk, it will react with the consumer of the drinks he is selling, changing the victim. Every time they have to expel gas as a result of this drink, they will change a bit more until they are a completely anthropomorphic version of their animal/mythical creature. It also works with some sweets he's selling. Evil, isn't it?
You stand in front of the evil hot dog stand with a pleased look on your face, relishing in the satisfaction that only proper hydration from a delicious beverage can bring. Suddenly, from within some deep partition of your body you feel a welling up of some unknown force. Fearing that perhaps your speedy consumption of the tasty cola has somehow shattered your coccyx, you wait with trepidation to see what will come of this roiling within your deepest being. Bubbling up from within, you feel it travel through your body and upward through your chest. Without warning, a breaking point is reached as the strage energy, refusing to be restrained by any biological barrier, rumbles its way directly from your chest and out of your mouth!
"Burrrp."
Looking at the owner of the evil hot dog stand, you sheepishly cover your mouth and mumble an apology. For his part, the evil scientist simply stares at you as if you've grown a second head. Thinking it rather rude that the man refuses to accept an apology for a common gastric expulsion, it suddenly dawns upon you that the neckhole of your shirt has suddenly become rather tight. Turning your head and trying to get a look at what is causing the disruption, your forehead smacks with some force into a rather hard object. Pulling your neck back as far as it will go, which for some odd reason seems to be a little farther than it should go, you come face to face with what for all the world looks to be a duplicate of your own face. Staring at this extra noggin which has quite rudely violated your personal space and, quite eerily, having it regard you with a stare of its own, you do take note of some differences with what you normally would see in the bathroom mirror, such as the slightly protruding fangs, longer neck, and mildly scaly skin that it possesses.