Why do they lie?

I had noticed that syrup bottle was under the dinning room table, open with chocolate over the floor.  "Isaiah!" I yelled sounding like Dave from Alvan and the Chipmunks.
A few minutes later Isaiah walks in.  "What mommy?"
Me-pointing to the bottle on the floor "did you do this?"
"No." Isaiah replied so quickly.
Me-trying not to laugh and hold my ground as a stern mom "Isaiah, did you open one of the new bottles of chocolate  syrup?"
 "No, I don't know who did it."

Now I have to stop here and tell you that not only did he have chocolate syrup all over his face, it was on his shirt, arms and legs.  So I walked him into the bathroom and had him look in the mirror.
"Isaiah-a lie is when you don't tell me what really happened.  So I need you to tell me again, did you open up the chocolate syrup bottle?"  Isaiah just smiled and said "Yes"

We go through this all the time with him and his sister.  Sometimes I know the answer and I am looking for them to tell me the truth.  Other times I don't know what really is the truth, so I don't know what to think.  Most of the time they pull this "she did it" "no I didn't he did it."  So what to do.

Children lie.  Some more than others.  When the are young they are still learning the difference between their world and the imaginary world they create.  Also at four years old kids are learning what guilt is.  Guilt is something that as it develops kids learn the difference between right and wrong.  They start developing a conscience.

I remember when my 10 year was about 6 or 7.  He was going to a VBS and he stoled a little toy plane.  He came to me that night and confessed that he had done this and was tormented.  To this day he still brings this event up in his life.  He knew he had done something wrong.  He doesn't still always tell the truth, but it doesn't normally take long for the truth to come out.  But some of my other kids, telling the truth is not always as easy.

Some kids miss these steps in their development.  Many kids that come from broken homes, on the streets or foster care- they miss huge developmental stages early on and since these stages are like building blocks when one is missed it is hard for kids.  Sometimes this leads to serve cases of Attachment Disorders.  Other times it just takes years of consistency of filling in those missing building blocks.

We tell all our kids that lying is not ok.  When you lie to us we lose trust in them and it is not always easy to gain back.  But for a younger child it is not about trust, but more on our part about training.  We are training our kids to teach them right from wrong.

What are ways you teach your kids your kids not to lie?