What do you say when your kids aren't being truthful?

Here are some ideas that I read will teach kids to tell the truth. When you think your kid might NOT be truthful about something, tell them:

“I won’t be upset with you if you did it – AND if you tell the truth, I’ll be really happy.”
The same author found another sentence that cuts down on lying by about 25%. Say to your kid, “I’m about to ask you a question. But before I do that, will you promise to tell the truth?”

Basically kids want to please the parent and knowing that truth pleases you they are more apt to do it :)