Yarn Bobbins Explained

Thanks to everyone who took the time to explain that the plastic thingamajigs are actually yarn bobbins which are used for times when you are knitting with more than one color yarn. No wonder I didn't know what they are used for...I avoid having to knit with more than one color of yarn at all costs! Although, now that I know the secret of the yarn bobbin, maybe I will try knitting with two or more colors at some point!