Fiona: I farted!
Me: Yes, you should say excuse me.
Fiona: No farts are for laughing.
Me: (biting the inside of my cheek) yes but you should still say excuse me.
Fiona: No! Farts are for laughing! Do you understand me?
And she looked so serious I of coarse lost it, cracked up, and completely failed to teach her a lesson about manners.