The difference between Truth and Lies

I am trying to teach Summer not to lie. She seems to always lie when she wets her panties. Here's the play by play:

Me: Did you pee in your panties?

Summer: No

Me: Tell me the truth.

Summer: I didn't.

Me: If you lie to me, I'm going to spank you.

Summer: I didn't pee in my panties.

Me: I will not spank you if you tell the truth. Did you pee in your panties?

Summer: Yes ma'am. I did.

Okay, so that is the basic scenario, minus the part where I try to explain that it is okay to have accidents, but you have to tell Mommy about them. No spankings for the truth, only for lying.

So, obviously we are in the process of learning the truth from a lie. But, I'm not sure she really gets it. At lunch she was telling her pink kitty cat to tell the truth. Not sure what it was lying about....