Why is this SO funny???

Payton thinks it is absolutely hillarious when Brian pretends to run into walls and falls down like he's hurt. Why is this? Why do children find it so funny when people pretend to get hurt?

It boggled my mind, until the other day when I recalled a memory from several years ago... We were living in a condo and Brian was BBQing in our back patio. When he turned on the grill a mouse jumped out of the bottom and went running for cover. Of course it surprised Brian and he went running too... right into the closed sliding glass door! I don't think I've ever laughed so hard in my life. How awful is that?!? Fortunately, he wasn't hurt... but I still chuckle every time I think about it. So, I guess it's not too odd that Payton finds this sooooo funny.... take a look...