A few years ago, when my now-7-year-old nephew was just 4 years old, we watched football one weekend. I tried to explain the game to him. At the end of the weekend, he explained football to me:
"Bad choices. Good choices. Watch the flag!"
Just yesterday, his brother - who coincidentally is now 4 years old - also explained football to me:
"You kick the ball and then you kick it again and again, and then you win. To get the trophy."
And there it is. What else is there to know?