What is the truth?

I have often had discussions with kids at school about honesty and being honest (usually when one or more of them are being DIS-honest!) They generally come up with the satisfactory definition of "telling the truth". Thankfully their parents have gone through the process of explaining what the truth is.

But what happens when you have to have that discussion yourself with a 3 year old? Where do you start?
"I didn't do it!"
"But Daddy saw you do it"
"I didn't"
"You aren't telling us the truth"......*blank look from Megan*
"You are lying".....*another blank look*
How do you explain what the truth is? The best we could come up with was "You said no when you should have said yes...that's naughty". Not the best explanation but the only one in kid speak that seemed to register with Megan.

Surely we will be able to explain better when she has a better understnading of things. I think she is pretty cluey and hopefully she will pick up on the fact that we wont tolerate lying. Hmmmm.....and I thought things would be easier once they could walk and talk!!