MOST LIKELY: You'll feel guilty. "Kids have incredibly sensitive systems for lie detection," says Dan Greenburg, author of the children's book Tell a Lie and Your Butt Will Grow , $5, www.amazon.com. "They will either know instantly that you're lying, or else they'll soon find out. You'll feel stupid and disloyal and guilty, and you'll end up apologizing. It's much easier to just tell them the truth in the first place." Your lies may also backfire. Tell your son that Rover will never come back because he's sleeping and he may fear going to bed for the rest of his life.

WORST CASE: They won't ever trust you, and worse, they'll follow your lead and lie a lot themselves. "Kids will have the same attitudes toward people that you do," says Richard Gallagher, Ph.D., director of the Parenting Institute at the New York University Child Study Center.