Mason watches everything, usually from here.
Not only does he watch, but he mimics. Cooper falls down, instantly Mason will lie down on the ground with him. Cooper uses the restroom, Mason lifts up his shirt and tries to pee with his belly button. Cooper cries or moans about some disciplinary injustice, Mason mimics the cry (but it's a fake of a fake and it's absolutely hilarious).
Surprisingly he hasn't used any curse words, yet. But I'm on the look out... for my little look-out... who's looking out for me.