Don't Show Your Long Finger...

So, during the girls' bath last night, Peter overhears Katie telling Delaney "Don't show other people your long finger."  Then proceeds to show Delaney her long finger.  So, I'm laying in bed last night resting and both girls run into our bedroom and tell me, "Mom, don't show people your long finger" as the proceed to flip me off!  I about died.  I ask them where they heard this, and they respond: "Dad" in unison.  Of course, Peter denies this, yet neither will tell us.  We talked to them about it and explained that it was very nasty and mean and it means a very, very bad word and if you use it, you can even be sent to the principal's office (to clarify how bad it is to them...we used terms they know and understand).  So, everyone...Don't show others your long finger...