As you drink the water, you feel a very odd tingling running all over your body, sort of like an electric shock. After a few seconds, nothing else of note happens, so you decide to go back home. As you walk inside, you notice a plate of designer cheeses, remnants of last night's party. Without thinking, you grab the whole plate and head up to your room, absently humming "Three Blind Mice" as you go, for no apparent reason. As you sit down in front of your computer, plugging away at your keyboard, you fail to notice that the pile of cheese is rapidly dwindling. After a while, you look at the clock and realize that it's 9:30 at night. Wondering why this is important to you, all further thought is driven from your mind, as your body is hit with an intense heat.
Not knowing how to respond to this sudden development, you do the only thing that makes sense, turn on the fan and take your shirt off. What you see doesn't so much as suprise you, as confuse you. All across your bare arms and chest, your skin seems to be squirming. As you look up in surprise, you see the same thing is happening to your face, and your legs if the discomfort is any indication. As you look back down, you see that white fur seems to be blooming out of your skin, and spreading all over your body, from head to toe. Your hair turns white as well, and when you raise your hand to examine it, you find that it has the same silky quality as the rest of your fur. "Your fur!" It seems so odd to attach a sense of ownership to something so alien. You start to laugh out loud, but close your mouth immediately, in case you woke up your parents. Raising a hand to scratch at an unpleasant itch somewhere on your chest, you stare in shock again at what is happening to your hand.
The bones seem to be swimming. As you watch, metacarpals shift around and your fingers shorten and your fingernails fall out to be replaced by claws. As you look at your hand, you notice that the fur hasn't covered it, it stopped at your wrist. Soon, both of your hands resemble rodent paws more than human extremities. You feel the same changes starting in your feet, and soon you have a matching pair of paws. Deciding that since you're already covered in fur, the need for modesty is kind of a moot point, you strip out of your pants and underwear to better examine yourself. You're covered from head to toe in pearly white fur, with the exception of your hands and feet. You've had a craving for cheese all day, even now you're cramming it in your mouth. And you seem to have shrunk a good six inches since all this started. You do the math, it appears something occurred today that has resulted in you changing into a mouse. You don't really seem bothered by this, after all, mice aren't so bad, at least you're not turning into an earthworm, or a fly, or something that would die within three days anyway. You have a good three years of life ahead of you, nine if that Roald Dahl book was accurate in any way*. Suddenly, you feel more changes happening, and you lie back in your desk chair to await them.
The first thing that happens is heralded by an unpleasant sensation that starts just above your furry buttocks. As you look back, a long bald mouse tail stretches out behind you. Twitching it experimentally a couple of times, you seem pleased with what you find. After all, you've sometimes wanted a tail, often when you had some kind of heavy load that you wanted to carry and someone else wanted you to hold something. Laying the tail in your fuzzy lap, you await the facial changes you know are coming, after all, you've already shrunk down to about 2 feet tall, and the rest of your body pretty much resembles a mouse.
As if to answer your call, whatever force (or farce) has done this to you takes the ethereal modeling clay to your face. The first thing you notice is an increase in your sense of hearing. Using your computer monitor as a mirror, you see that your furry ears have flattened out, seem to have moved to the top of your head, and have gotten bigger. They now resemble a pair of living satellite dishes on top of your head. You reach up and touch them, just to be sure, and once you are satisfied, you assume your original position. Your nose and mouth push out into a muzzle, and whiskers sprout out on either side of your nose. You resume stuffing your face with cheese, now able to savor it more because of the stronger signals your new nose gives your brain. Stopping once more to look yourself over, you find that you have shrunk down to about six inches tall, a little big for a mouse (at least going by the ones you've seen), and are pretty much a complete mouse, except that you can stand up on your hind legs with no real effort, and your eyes, which you see once you climb (you DON'T scurry) onto your desk, are the same color they always were. You decide to test your new vocal cords, and try to say your name. It works, but your voice is a little high and squeaky. Running your paws over your soft and silky new mouse body, you don't really see any other anomalies that set you apart from any other mouse. Deciding that these changes have taken a lot out of you, you climb over to one of your desk drawers to bed down for the night, because if you sleep in plain sight, your parents might notice you. Dropping down into it, with some difficulty you are able to close it so it isn't too obvious. Curling your tail around yourself, you drift off to sleep and dream mouse dreams.
*Roald Dahl's The Witches features a kid who turns into a mouse. Later on, it is revealed he'll live for 9 more years. Kinda morbid, no? It doesn't annoy him much because he really didn't want to live much longer than his grandmother.