Children tell lies for lots of reasons. They want attention, they are jealous, they feel left out, they are cross with someone, they don't want to be punished for something. They grow out of it most of the time. I used to put mine on the naughty step which worked with the older two, but the youngest wasn't worried at all. I caught him out one day and on the naughty step he went; After five minutes I felt sorry for him and went to see if he was alright. There he was eating a bar of chocolate he had found in the kitchen cupboard! I had to use other tactics after that. He has grown up to be a lovely boy, you could not wish for better.