We've all been in a similar situation in the past. Mom walks into the kitchen and finds evidence of unauthorized snacking. The inevitable questions come. “Who did this?" “Was it you?"

If you're the guilty one, you have two options: tell the truth and face the consequences or try to tell a convincing lie to avoid the consequences. We urge you to tell the truth, because lying isn't a good habit to develop. Besides, most parents have a built-in sense that tells them when you're not being truthful!

Law enforcement professionals, such as the police, often need to know when someone is telling the truth or not. Since they can't rely on parental instincts, they sometimes turn to a special tool called a polygraph machine.

A polygraph machine is more commonly known as a “lie detector." You've probably seen polygraph machines used in movies or television shows. You may have wondered whether they can really determine when someone is lying.

Polygraph machines measure certain bodily functions, including skin conductivity, heart rate, blood pressure and breathing. These are functions that can't be easily controlled by the mind. Many people believe measuring these functions can reveal when a person is not telling the truth.

A polygraph test usually involves asking a person hooked up to a polygraph machine a series of questions. Some questions are control questions. These are simple questions with known answers, such as the person's name and address.

A baseline reading is established based upon the control questions, since the examiner will know the answers are truthful. When the key questions — questions the examiner doesn't already know the answers to — are asked later, the responses can be compared to the answers to the control questions.

If the functions being measured differ from the measurements taken in response to the control questions, the examiner will likely conclude that the person being tested isn't being honest. But how reliable are these tests?

Critics of polygraph machines believe that they aren't reliable, because there are too many variables involved to be a valid scientific test. Many courts agree with the critics. Polygraph results are often not admissible in court proceedings. In areas that do allow polygraph results, they are often only allowed to be used in limited ways and not as proof that someone is lying.

Researchers have found that polygraph machines may detect lies in many cases, but that they may be susceptible to errors, too. For example, some people have shown that they can use certain strategies and techniques — called countermeasures — to fool polygraph machines.

So could you beat a lie detector? Maybe! Common countermeasures include trying to increase physical responses during the control questions. This could take the form of purposefully increasing your breathing rate or pinching yourself. The idea behind these types of countermeasures is to establish a false baseline on the control questions.

Others claim they can beat a polygraph machine simply be getting a good night's sleep and remaining calm throughout the examination. Some people mentally repeat the control questions to themselves as they answer the “real" questions. Doing so may help to control their bodies' involuntary reactions.

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