"...lab rat. You'll be too smart to use for the average conditioning experiment, but I guess I can still use you for a variety of other fun things. Not in the last place a study how well you'll adapt to living in a cage."
You are confused at the man's words and begin:"Lab rat? What are you twalkwing abwout--" when you suddenly notice talking is getting more difficult. The man grins. "Ah, your teeth have already started to grow. Very useful for eating the daily diet of food pellets ahead." Suddenly you go crosseyed in surprise as you see your own face extending out in front of you a bit, pairs of whiskers bobbing on the side as you move your new muzzle. You don't manage to see it for long though, as your vision moves to the side a bit and your ears curl into small round rat ears. You squeak in protest, but the man just laughs. Across your entire head, there is a thick cover of white fur now, which spreads to the rest of your body as the changes continue, hands first. They have a very bony appearance, and your fingernails have become much harder and sharper, turned into claws.
You try to run away, but trip over your oversized clothes because you're shrinking at the same time. He chuckles at your escape attempt, and closes the window and door to prevent future attempts. Your clothes flumping to the floor from your shoulders, you can see that the white fur has spread across the rest of your body and your feet now approximately look like your pawlike hands. Your shifted body configuration and weight forces you into a quadrupedal stance, which is now even more comfortable than standing on your hind legs. Finally, your spine feels like it's extending behind your back as a long pink rat tail starts growing above your butt, lashing and curling behind you within moments. The man who just changed you now seems gigantic, and before you know it he picks you up by grabbing you across your body and promptly dropping you in one of the open-top glass cages on the tables. You frantically try to climb out while squeaking in despair, but it's no use. Your forepaws have no grip on the smooth surface, and you're stuck in a rat cage.
He chuckles, drops a piece of cheese in your cage as a treat and walks to leave. "I'll let you get used to your new home for now, I'll be back later for more...experiments." He slams the lab door shut upon leaving and you're left wondering what to do, nowhere to go beyond the cage edges.