Using their heads.

[greenheadmachine]

A Bloomington, IL policeman had a perfect spot to
> watch for speeders, but wasn't getting many. Then he
> discovered the problem - a 12-year-old boy was
> standing up the road with a hand painted sign, which
> read "RADAR TRAP AHEAD". The officer then found a
> young accomplice down the road with a sign reading
> "TIPS" and a bucket full of money. (And we used to
> just sell lemonade!)