Memo to a carful of teen-agers out in Fairfax: knock it off before you hurt someone.

Gloria Noon's call the other day was only the most recent in a summerlong series. She was the victim of a balloon full of water, tossed from a car that passed hers at a high rate of speed in the Middle Ridge subdivision.

Anyone who didn't sleep through physics class knows that a balloon chucked from one car going 60 miles an hour at another car going 30 in the opposite direction develops an enormous amount of force.

In Gloria's case, the force was great enough to smash the windshield. Her husband was driving, and he was able to get control of the car before it sheared off a tree or two. But it could have been much worse.

The Noons could not see the occupants of the other car; as the victims always seem to say in cases like this, it happened so fast. But Fairfax County police say they have been besieged by identical incidents ever since the July 4 weekend. In every case in which the victims have seen the ballooners, they have described them as teen-aged males, police say. No one has been hurt, police added, but no one has been arrested, either.

Now that school has reopened, the ballooners just might pay enough attention in physics class to figure out the risks they're triggering.