Both my boys started saying "What the heck?" out of the blue.  This isn't a phrase I use nor do I think it's appropriate for 3 year olds to utter.  It's such a habit they say it without realizing it until it's out.   My typical response is the stink eye, a reminder that it's not appropriate, and a thinly veiled threat.  It continues to happen.  Next time I hear it I'm literally washing the perps mouth out with soap.  *sigh*  Grrgh!  What the...  ?!