A (computer) monitor
Some can be forgiven for thinking I watch you,
but in fact you are the one watching me.
A monitor can be one who watches, but in this case, I'm looking at my monitor right now
I suppose I keep an eye on the mathy fellow,
but that just makes me a little bit gooey.
Not entirely, sure, but I'm guessing the mathy fellow is the CPU?
I guess I don't lead but I can be led,
though sometimes I have crystallizing moments.
Or liquid crystal monitors
without light I am out of sight however,
You can't see anything if the monitor produces no light
with a magnet I can get real funky.
Especially if you're a CRT monitor, as magnets will distort the picture
To give your answer you would key, but you would get little feedback without me.
I won't know I'm right until I see a response to this on my monitor :)