Can Adults Become Human?

Read the hilarious, candid (and sometimes not-so-nice), diaries of Jamie Kelly, who promises that everything in her diary is true...or at least as true as it needs to be.
Dear Dumb Diary,
My social studies teacher, Mr. VanDoy, never smiles. I know that's hard to believe, because everybody smiles about something, right?
Isabella smiles when her brothers get in trouble. Angeline smiles when she thinks about how much prettier she is than, like, a waterfall or a unicorn. I smile when I think about a unicorn kicking Angeline over a waterfall. But Mr. VanDoy doesn't smile at all. I wonder if when you become an adult, you can lose your sense of humor the way you lose your teeth or hair or fashion sense.