Aka a water gun. Squirt gun can sometimes imply a diminuative water gun. It is a plastic hand-held (usually) weapon that fires a stream of water. They range in size from tiny keychain-sized ones to large stationary emplacements. The midrange ones look like something out of a sci-fi movie; they have a very obvious good end and bad end, as in the person on the bad end of the gun knows it. Some kinds have a plastic backpack water reserve that holds a couple of gallons at most. These look like the things that the ghostbusters use.