Cow Giving Birth_英语笑话

a man was helping one of his cows give birth, when he noticed his 5-year-old son standing pie-eyed at the fence, soaking in the whole event.
the man thought, "great... he's 5 and i'm gonna have to start explaining the birds and the bees. no need to jump the gun - i'll just let him ask, and i'll answer."
when everything was over, dad strolled over to his son and said, "well willie, do you have any questions?"
"just one," gasped the pie-eyed lad. "how fast was that calf going when he hit that cow?"