In the car with my 5YO daughter this afternoon.....
Daughter:  "Dad, why do you always say "Um hum" when I tell you something?"
Me:  "Well, that's my way of letting you know that I heard what you said."
Daughter:  "You could say something else every once in a while."
Me:  "You're right.  I definitely could say something else sometimes.  Sorry, it's a bad habit."
Daughter:  "Dad, you don't need to apologize.  I'm not angry with you.  I just think that you could say something else.  I mean, I know you have a brain, right?  So, why don't you use it and think of something else to say?"
Me:  "Got it.  I totally agree with you.  Thank you for telling me this."
Daughter:  "No problem.  I love you, Dad."
Two minutes later.....
Daughter:  "Dad, how long until we will be home."
Me:  "Almost there.  Like two minutes."
Daughter:  "Um hum."
I look into the rearview mirror and see her smirking at me.  We both bust out laughing.  Needless to say, kids are sometimes not only hilarious, but also very insightful.