2006 Pony Pkg cooling fan question

I just got the car two weeks ago, and I'm noticing that the cooling fan (attached to the radiator) comes on soon after the car is started (if you let it idle). I don't idle that long...sometimes have to run back into the house quick to grab something...and when I come back out, the cooling fan is running.
I've checked (and topped off) the coolant in the resevoir, and I don't have a leak. is this common? the temps here haven't been that hot lately, and the car's temp gauge is reading a normal temp (right about in the middle range of the gauge). Any ideas why the fan is coming on so soon? :headscratch:
Thanks in advance,
Jay