I tell Elli untruths constantly. I don't think they are appropriately called lies because I have no intention of actually fooling her but I definitely spend a lot of time saying things that are not true. She was fussing about her legs being cold after her bath and I talked about trading them in for a new pair of legs. She didn't believe me at first, but when I told her that the new leg store was beside Canadian Tire she stared at me wide eyed and asked "Really?" No, not really, but it was fun to get her to imagine what colour of legs she would buy there. She settled on legs of exactly the same colour as the current ones because she doesn't want to have different legs - reasonable enough.
I do this for a few reasons. First off it is a lot of fun for me to try to tell her untruths that are right on the edge of her understanding. If I tell her that the moon could fit in her pocket, she knows that I am fibbing. If I tell her that it is about as big as her school she really doesn't know the truth of the matter and has to try to sort out if I am telling the truth or not. Normally I would assume that she could easily realize that there aren't any leg stores out there but the added 'fact' of the store being near to Canadian Tire pushed her over into being unsure. It is a game I play to try to say something that she can just barely figure out isn't true and I enjoy trying to hit that mark.
The other reason of course is to teach her to never believe anything anyone says on the basis of 'somebody said it so it has to be true'. Clearly we have to accept people's word on small things because we can't fact check every statement but when important decisions are being made you have to assume that people might not be telling the truth. It is important to develop a healthy skepticism of things people say that just don't seem right, and also a little bit of doubt about things that *do* seem right. I want her to know that I will always let her in on the joke but that even when someone in authority says something in a utterly certain tone it isn't necessarily correct. Of course when that person is running for public office you can usually assume that anything they say that seems squirrely is entirely false; see leadership debates in pretty much any country.
It is funniest when I end up telling these sorts of whoppers to other kids. Apparently not all parents have fun making things up to see if their kids can tell what is true and what is not so when I try this with other kids they nearly always fall for it hook, line, and sinker. I have had to apologize to other parents when their extremely credulous kids got some nutty idea into their heads and refused to believe it was a joke - so far they have always thought it was funny but someday I am going to dig myself into a big hole. Hopefully what Elli takes away from this is the habit is the knowledge that the sincerity of a person's statement has nothing to do with the truthfulness of it. Truth is tested by science, not zealous presentation.