An off-duty police officer, familiar with radar guns, drove through a school zone within the legal speed limit when the flash of a camera went off, taking a picture of his license plate. The officer, thinking the radar was in error, drove by again; even more slowly. Another flash. He did it again for a third time, at an even slower speed. Same result.
“This guy must have mixed up the settings,” the off-duty officer thought.
A few weeks later, when he received the violations in the mail, he discovered three traffic tickets: each for not wearing a seat belt!
How do you know you are out of invisible ink?