Father and son joke How to open closed the door

There was a little son who asked his dad about his assignment
Little son: dad! what is escalator? 

Dad: oh! very simple my son, ,it is stairs go up and down
Little son: asked by dad!  

what is elevator? 

Dad; again very simple That is door open closed goes up and down Little son:  what is calculator? 

Dad; oh! very simple it is a door open closed like you brain for paying school fees with extra charges 

Little son: I haven'tsee that going up and down even close or open. Neither I have seen door open closed,  as my mom gave me birth but only see you while going up and going down on my Mom

Dad; oh! very bad why you like this for old books.You want to be dirty boy 

Little son:  No no Dad! you know any definition is cleared and explained with photos only in old books. That is why I am always looking at you and looking at Mom whats new formula  for love subject how open closed the door before enter inside it