Lying

Our kids are getting adept at lying. I am not sure I like it.
The lies are usually fashioned this way:
Me: B, did you hit Z?
B: Um....what will I get if I did hit Z?

S: MOM! Z took my toy that I was playing with!
Z: No. It is mine! I was playing with it first.
Me: When were you playing with it, Z?
Z, in a mumble: Lasterday (yesterday).
Me: Good try, Z. Give it back to S.

S, crying and pinching her butt cheeks together, like she always does when she has to poop: Eeeeee! (Running to hide), Errrgh, (still pinching those cheeks together).
Me: S, go poop. Right now.
S: I'm not doing that noise because I have a tummy ache and have to poop (she knows the drill). I have a headache. That's why I said, 'Eeeee!'
Me: Go poop.