I will definitely be looking up your 3 stooges website, Stoogewoman!
The cranks are the part of the bike that attaches the pedals to the bottom bracket. This is a most critical part, a part that has to bear your weight when you are pushing hard on the pedals and standing up on the pedals, like when you are accelerating or going uphill. You definitely do not want this part to snap in half, possibly breaking your leg, pelvis, whatever.
Maybe I'm not up to date on the latest Walmart junk bike technology, but i've never heard of plastic cranks before.
Parts of a bicycle:
http://www.jimlangley.net/wrench/bicycleparts.html
(They call it crankarms there)