Because "Sun"? That's just too Egyptian.

Isaac requires that everything have a name. Not just people, every THING, too. He'll pester you without relent until you come up with a person-style name for every creature or object he comes across. Here is the typical dialog.

Isaac: What's this?
Mommy: (playing dumb) I don't know. What do you think it is?
Isaac: That's a cow. Know his name?
Mommy: Uh...uh...Clarence the Cow.
Isaac: (with complete certainty) Clarence the Cow. You right, mommy.

That crab? He's Carl the Crab. The squishy toy frog? Freddie the Frog. Our car? George. George the Green Car. Sometimes I'll mix it up and go with descriptors instead of "names", such as Spot the Dog, Stripey the Fish, or Fluffy the Kitty Cat. This appeases him also. I know I'm not the brightest apple to fall from the tree, but you would be surprised the mental effort it takes to name every single thing in sight something different, and also something different from characters we know from TV and real life.

Yesterday we were driving home after picking Daddy up from work, and the sun shone its pain-rays down on Isaac in his car seat.

Isaac: Daddy, the sun's burning my eyes.
Daddy: Oh? Are you okay, buddy?
Isaac: Yes, I okay. (pause) Know his name?
Daddy: Whose name?
Isaac: Sun. Know his name?
Daddy: Buddy, the sun's name is just "Sun."
Isaac: Know his name?
Daddy: Sun. "Sun" is the sun's name.
Isaac: Know his name?
(Mommy and Daddy ignore endless-loop Isaac for a second to chat together)
Isaac: Barry. Barry the Sun. Right!