Spelling

My first grader just asked me to spell "unobtainable."
"U-N.."
"Yeah..."
"O-B.."
"Yeah..."
"T-A--"
"T-A-I-N," Aharon says.  (Whoa, I think.  He already knows how to spell most of it.) "I-B-L?" he asks.
"A-B-L-E," I say.
"You got it wrong, Mommy," he says, a moment later.  "It is I-B-L-E."
"What are you talking about?" I say.  I google it. "Come look at the dictionary.  Unobtainable." 
He comes to look.  "Well, that's not how they spell the code."