"Boy, I'm hungry," You think "Let's go to the hot dog stand over there."
What you don't know is the hot dog vendor is an evil mage with a crazed obsession with transformations, especially fictional ones. He has concocted a wicked plan that when one of his hot dogs are eaten, they will react with one 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?
"Hello sir." He says "Want a hot dog?"
"Yes please." you say. You eat your hot dog.
"Mmm, that was lovely, but now I'm thirsty. I think I'll have a drink of..."