Your son swears up and down that he didn't take the car, yet his eyes don't meet yours. Your daughter tears up and rubs her nose when she tells you -- "Honestly, Mom!" -- that the cigarettes aren't hers.

Are your kids lying? Probably. The truth is everyone lies, says Dr. Alan Hirsch, a neurologist and the author of How to Tell if Your Teenager Is Lying. "We were born to lie; it's part of what keeps the species going," he says. "What this means in evolutionary terms is a selective advantage among those who are the most successful liars."

Not surprisingly, kids lie more than most people. A study by Kang Lee at the University of Toronto's Institute of Child Study found that 20 percent of kids fib by age two. By 16, 70 percent agree they lie -- at least those are the ones who admit to it.

So how do you tell if your teen is playing fast with the truth? According to Hirsch, there are several key "tells."

The Pinocchio effect
During a lie, the sympathetic nervous system triggers a fight-or-flight response that inflames the nose's erectile tissues. The result? Former U.S. president Bill Clinton touched his nose once every four minutes when he lied to the U.S. grand jury.

Clammy hands
As the lie progresses, the fight-or-flight response also draws blood to the centre of the body, creating cold, clammy hands.

Liar's lean
During testimony, Clinton leaned forward nearly three and a half times every four minutes. "Your body language is saying 'Come and be an accomplice,'" explains Hirsch.

Expansion of contractions
"I did not eat the cake!" may be what comes out of your teen's mouth, but the expansion of the contraction "didn't" is a sign he gobbled the gâteau. "By emphasizing the 'not,' you're convincing them and yourself that it didn't happen," says Hirsch.

Qualifiers
Known as the partial truth, using phrases such as "not necessarily," "sometimes" or "honestly" indicates that, while there's a grain of veracity, it's hidden under a mountain of maybes.

Trapping the lie
Ever notice how liars will take a sip of water or cross their arms and legs? Hirsch says that's because "the lie wants to come out. They feel guilty and want to purge themselves of the guilt."

Photography, iStockphoto.com.