Christopher's Sentence

Our 4 year old son Christopher will turn 5 in November. Last night he came into the room with a drawing he made and a sentence he wrote. In case you can't make it out, it says

"iEMUBTTRNFiF 5"

Which translated says "I am about to turn five."

I was amazed. I asked him, "who told you how to do that?"

He replied "just all by myself.

I said, "but how did you know what letters to write?"

He said, "I just put it into my mind and then wrote the letters."