How do you handle a child that lies?

Today's example

My son came home from school and said that he had no snack in his lunch today - i packed his sandwich and snack last night and they just had to be put in the lunch box. So i asked him again he said he had no snack and they had to give him a snack at school.

My so said that he gave the snack and sandwich to my son who packed it...

So I asked my son again and he admitted to lying and making it all up.

Normally I wouldn't question him but i knew it was packed etc...

Tired of being lied to