With your eyes nearly bugging out of your head and your mouth hanging open, you stare into the mirror; the concept that you have spontaneously morphed into a teenage girl causing your brain to briefly go on strike. This undignified gaping stretches for some time before you manage to shut your mouth and look down at yourself in an attempt to verify what the mirror has shown you. To your continued shock, you find that your underclothes do indeed hang loosely over your now-petite form. Upon pulling the band of your boxers away from your hips and taking a good look within, you take note of what you would consider to be a very large (though entirely average by any objective scale) absence, and the disturbing idea of being stuck as you are suddenly starts to float about in your imagination.
Opting not to do anything so stereotypical as crying, you instead gather your wits and peer with an accusatory eye at the mirror, which you deem to be the most likely cause of your change. Given that the only thing which you had done upon viewing the mirror in the first place was think about what you might change about yourself, you fix it with a determined stare and picture your regular body again. Almost instantaneously you feel yourself growing, and the scene in the mirror bears this feeling out as the reflected image of a young girl morphs back into the far more familiar sight of your own body.
Quite relieved to find that you will not soon be filling out, getting 'those looks' from the boys, and having a little 'monthly visitor', the knowledge that you can use this mirror to change with no apparent permanent consequences sets your mind racing. Would it let you impersonate the president? Could you take on the appearance of your hot co-worker's boyfriend so that you can engage in a little lunchtime tryst with her being none the wiser that she is actually cheating? Or perhaps it would best serve by making a few little generic athletic modifications? The last idea being the most easily tested and immediately useful, you look into the mirror and think about all of the little tweaks you'd like to make to your form. A little more muscle here, a little less fat there, and a tiny bit of retouching at the hairline causes the reflection to match what you might look like after several months of vigorous exercise and much healthier eating than you generally opt for. Giddy with the idea that you could walk out of the mansion with a much improved physique, the curiosity over what the upper limits of the mirror's capabilities were jumps back into the forefront of your thoughts and the idea of experimenting a bit follows quite naturally.