Man, I probably have a zillion stories like this if I stopped and thought about it. The one I clearly remember happened when my daughter was five and my son was two.
We were playing "School" and my daughter was the teacher and my son and I were the students.
Daughter: "Does anyone have any questions?"
I raise my hand.
Daughter: "Yes Daddy, what is your question?"
Me: "What is the meaning of life?"
Daughter: "Daddy...I don't know!"
Just then the two year old turns to me (he was sitting next to me), grabs my face and turns me toward him so we are looking eye-to-eye. He has this serious look on his face.
Son: "I know."
Me: "You do? What is it?"
Son: "Applesauce!"