My sister told me to tell my children that I know when they are lying because their cheeks puff out. So now when they begin to tell a big fib they try to hold their cheeks or suck them in or just become very self conscious of their cheeks. She's a genius.
You can watch this old video of Jane. She is trying to convince me that her teacher wants me to buy her new play dough. It really doesn't matter what she is saying, just watch her struggle to get it out.
Liar, liar, pants on fire.