"Front Runs"

Every child at some point learns to call "dibs," or "shotgun," or whatever term they are taught. However, I did not expect my three and two-year-olds to come up with a term on their own, and attach to it the normal rules associated with say "shotgun" (the rule being-whoever calls it first gets it without any further debate).
I noticed this when the kids were racing trucks around the house and Audrey suddenly shouted "Front Runs" (or "Front Ones" it is kind of hard to distinguish between O's and R's in a toddler's vocabulary) and Derek immediately relinquished his coveted dump truck. A few minutes later Derek yelled "Front Runs," and the trade off occurred again. Since then they have been using it for everything. Where do kids learn these things?