The aim is to call "Chubby" on a van before another person and call more chubbies than another person.
It is a good game for a long car trip. You may even like to tally points or play where you get to punch someone if you call chubby and get punched if you accidentally call chubby to a fake chubby.
Dave: Nah man! That's fake Chubby.
Frank (spots a Mitsubishi Express van): Chubby!
Clive: Damn it!