What goes of at an airport?

Question

I was wondering what makes the xray machine beep at an airport. You know those scanners you put your bags through.

Answer 1

http://www.tsa.gov/travelers/index.shtm

This website should help you figure out the answer to your question.  

Answer 2

The x-ray machine where your luggage goes doesn't beep.  Each bag is examined by an agent, and if they see something suspicious they will ask to search it.  

When you walk through the arch, if you have metal on (belt buckle, jewelry, body piercing, etc.), it will probably set it off.