My wife used to have the habit of disbelieving something with 
the phrase "my ass!"  She would say "Four hundred dollars, 
my ass!" or "30 minutes late, my ass!"  One day a friend of 
mine and I were having a conversation, which she was listening 
to, and I said something like "...so it would be easy to 
penetrate.."  She chimed in... "Penetrate my ass!"  My friend 
and I laughed so long and hard we forgot what we were even 
talking about!  Needless to say, she doesn't say "my ass!" any 
more.