Me Cow.

Brian: "Cow cow cow."

Me: "Cow? Where's the cow?"

Brian: "Me cow."

Me: "You're a cow?"

Brian: "Moooo" giggle giggle giggle

He was seriously laughing his butt off with a gleam

 in his eye after saying this.

Brian's first joke?

Or even more astounding:

A new pretend play skill?

He has NEVER pretended to be anyone (or anything) before.