I used to hate it when my Mom did this to me...

and now I'm doing it to my kids!! The boys will be upset about something or getting angry, whatever it may be and I will tell them that they better not smile/giggle. That it's not allowed, etc. I say it with a really angry face and they both crack up hysterically. My Mom used to do it to me all the time and it always pissed me off because I could never help but smile. I guess my kids are weaklings too.