Polygraph Testing

A polygraph, or "lie-detector," is a machine that measures various physical attributes (blood pressure, perspiration, heart rate, etc...) and attempts to correlate them with telling lies. In past times they were often used by police and government agents to interrogate suspected criminals, but as they have been proven to be extremely unreliable indicators of lying, their use has lessened in recent years (usually because of court rules prohibiting their use).  

In fact, there is no state that is legally allowed to force you into taking a lie detector test, although the police can usually force people into believing not taking the test is an admission of guilt. 

Federal courts have their own rules on when to allow the admission of polygraph evidence (it is usually at the judge's discretion). For a complete list of the state and federal rules regarding polygraphs, you can check here for your state.