My older brother, then age 12, was getting a physical.
The doctor said, “Now start breathing.”
Gary asked, “When have I stopped?”
Many years later our Mom was irritated with me about something and asked me, “Can I ask you a question?”
I said, “That’s a question already.”
My older brother was present; I’m lucky neither one of them threw something at me!
I was working with that German carpenter when, about 11:30, I asked if he wanted to go to lunch.
He said he wasn’t that eager to eat. He asked, “Do you think I’m a horse?”
I shot back, “No! Your ears are too short.” (Lucky for me he had a good sense of humor!)
Please post here something smart-aleck said in your family, that you remember. 
After my parents and I ate at a DC Chinese restaurant, we got our obligatory fortune cookies. Mine read “Wisdom is your middle name.”
I showed it to my mom and said, “You told me the ‘W’ stood for William.”
I looked in the mirror today/My eyes just didn’t seem so bright
I’ve lost a few more hairs/I think I’m going bald - Rush
My (then three year old son) said: “Mommy you don’t have a penis”.
I: “You’re right, I don’t have a penis.”
Son: “Only Daddy and me have penises.”
Me: “You’re right, only boys have penises.”
Son (turning to daddy with hand in the air): “Daddy,High-Five Penises!”
After working for years in the airline industry. It’s made very clear that there are no stewards or stewardesses, they are ‘flight attendants’. Anything else would be demeaning. My 3 y/o daughter put all this “politically correct” crap in perspective. After we boarded the plain and were seated she said in a loud voice, “Daddy, where’s the peanut girl?”
Reminds me of Mel Brooks… “Oh, Piss Boy”
These are pretty good!
Back when I was a kid, my dad was kvetching about taxes, and said “we don’t get shit back around here!” My brother, walking by, didn’t miss a beat as he replied “you do if the toilet stops up!”. This broke my dads’ rant instantly! 
VB
“Hey! How 'bout that Toe Jam?”
My wife is prone to seizures. One unfortunate day, she had a seizure at the top of our stairs, and tumbled down. In the process she shattered the bones in one foot, and broke her other ankle in a couple of places. ANYWAY, when the paramedics were there working on her, she had been quiet all along, when she suddenly started screaming in pain as they were moving her from the floor to the gurney. “Where are you hurting?” asked one of the paramedics? “You’re standing on my hair” cried my wife. The paramedic at her head was accidentally standing on her hair.
My parents were talking about a ventriloquist who was extremely famous in their childhood (the older Dopers will no doubt be horrified that I don’t know the guy’s name :)). They mentioned that he had a popular radio show. My sister asked “What did he do? Make his voice sound like it was coming out of the toaster?”