the act of the drive-by watermelon is as follows:
-head to a local hangout frequented by chavs/general delinquents (McDonalds car park is usually a good location)
-pick out an individual deserving enough to receive the impending assault
-slow down the vehicle and wind down the window, then lob said watermelon squarely at the recipient's groin
-you may need to speed up and escape at this point to avoid retaliation, however the watermelonee will most likely be on the floor in pain, and his acquaintances will generally be too confused and too busy laughing at their unfortunate chum to raise a counter-attack.
-retire to a safe distance