"Inexplainable" is a word in ENGLISH

A student called up his Mom one evening from his college and asked her for some money, because he was broke.His Mother said, "Sure, sweetie. I will send you some money. You also left your economics book here when you visited two weeks ago. Do you want me to send that up too?""Uhh, oh yeah, O.K." responded the kid.So his Mom wrapped the book along with the checks up in a package, kissed Dad goodbye, and went to the post office to mail the money and the book. When she gets back, Dad asked, "Well how much did you give the boy this time?""Oh, I wrote two checks, one for $20, and the other for $1,000 out to him.""That's $1020!!!" yelled Dad, "Are you going crazy???""Don't worry hon," Mom said, kissed Dad on the on top of his bald head, "I taped the $20 check to the cover of his book, but I put the $1,000 one somewhere between the pages in chapter 15!"