How do you keep airport scanner operators alert?

"Every TSA X-ray machine has a Threat Image Projection system, which digitally inserts images of guns, knives and bombs into the X-rays of luggage, to keep screeners alert. This system library contains "tens of thousands" of images, said TSA spokesman Christopher White.

"If screeners observe a suspicious object, they can check with the simple click of a computer mouse. If they detect a threat object, the computer congratulates them. Successes and failures are recorded for use in a screener's performance evaluation and are factors in determining pay."

Source: http://edition.cnn.com/2008/US/01/28/tsa.bombtest/index.html