"Little Johnny on Sex Ed" joke

Little Johnny walked into class one day and sat down. He realized that someone new was teaching the class today, It was Coach Bob, the gym teacher.
Coach Bob started the class by saying, "Okay you little one's are never too young to learn about s-e-x, uh ummm."
While every other kid in the class is giggling. Little Johnny looks really intent.
Coach Bob then asks, " Well, what do any of you kids already know about sex."
Little Shirley raised her hand, "One time my puppy had a baby."
"Good " said Coach Bob
Then Little Chuckie raised his hand and said, "Well my mom gave me a little sister."
"Good, good." said Coach Bob
The Little Johnny raised his hand and Coach Bob's heart skipped a beat. He said "Yes Little Johnny."
"Well," said Little Johnny, "one time at my grandpa's house I was watching an old western starring the Lone Ranger, and he fought off one hundred Indians."
"Oh Yeah," said Coach Bob really relieved, "what does that have to do with sex ed."
"That'll teach those Indians to screw with the Lone Ranger."