A man was helping one of his cows give birth when he noticed his 4-year-old son standing wide-eyed at the fence, soaking in the whole event. 

The man thought, "Great, he's 4 and I'm gonna have to start explaining the birds and bees.  No need to jump the gun, I'll just let him ask, and I'll answer."

After everything was over, the man walked over to his son and said, "Well son, do you have any questions?"

"Just one," gasped the still wide-eyed lad.  "How fast was that calf going when he hit that cow?"