Shoulder Shrug

Recently Matthew has taken to using what is probably his first phrase: "I don't know". Every question receives the same response.

"Matthew, did you sleep well?"
"I dooon't know" (Large shoulder shrug).

"Matthew, do you want pancakes?"
"I dooon't know" (Large shoulder shrug).

"Matthew, where are your toys?"
"I dooon't know" (Large shoulder shrug).

I don't know where he got this. (Large shoulder shrug).