Those guns are also available in soap form also. Just judging from the fact that the little girl is wrapped in a towel with wet hair, this is probably the more likely scenario.
As disturbing as this might be to some, I can see where in some situations this could be considered not so bad as many would make it out to be. If it were in fact a soap gun, perhaps the sister thought it funny that she would "blast the dirt away" or something to that effect.
Much more disturbing to me is the current trend to video pranks designed to scare the crap out of random people in the streets. One I have seen was of a person dressed up like the Chucky character from the horror movie wielding a knife. He busts out of a sign and chases people down the road. This, I think, is an even bigger problem. If someone busts out of a sign waving a knife at any one of us here, I am pretty sure the majority would result in a very sad ending of something that was intended to be funny.
