5 years old must mean she has exposure to other children in preschool, day care etc right?
Usually kids begin to "lie" because of fear - of getting into trouble, being punished physically, restricted from fun etc what ever the negative in a 5 y/o life is the 'fear factor' is there.
Try talking to her when there is NOTHING going on at the time to see what her concept of a lie is.
I have found that the adults name the game so to speak. I know you did not teach her how to lie so at 5 years old how did she learn 'it' - We call it a lie what do kids call it? Find out what she knows before you attempt to correct it - usually the answer is there in what she learned.
Here is an example of a conversation I had with my 5 y/o grandson who actually tells lies to mom about me (lol little bugger lol)
"Hey there Tom, I have a question.....when you told mommy that you and sissy took a bath together the other night I was confused" "Sissy was playing with sam in the kitchen" so can you explain to me - how you thought she was taking a bath with you?"
"I know I didn't mean sissy I meant Lindsey she used to take baths with me when I was little"
"Oh so you remembered it and said 'sissy' instead of 'lindsey'?
"yeah, I get confused sometimes"
"huh, that's cool so do I" "Thanks I just wanted to know how it got mixed up" "So what did mom say (smiling)?
"She said I was too big to be taking baths with girls and I was not to do that"
"She's right- I totally agree, maybe next time you could tell her you meant "lindsey" instead of 'sissy' and she won't worry - how bout that?"
"yeah I just got my names mixed up"
"it happens, don't worry about it"
We continued to play with the playdoh and knowing that he was simply 'incorrect' but that his mom's reaction was a bit stronger than the situation merited - I had a talk with her and explained that sometimes we hear more than what is actually said or emphasize it.
Explore what the child is saying in what setting and why - then ask if kids in his class get things mixed up to and how does that happen?
Kids have basically got to be taught what is 'correct story' and incorrect story - the concept of lieing is too vague for their cognitive abilities - we assign the 'lie' to the task and then the consequences but we are not usually age appropriate in our doing so. We expect a more abstract ability when they are only capable of concrete thinking.
There is different levels of maturing in their develpment according to Piaget there are:
It is facinating reading and helps a lot when you teach understanding what level the childs is at.