Knocking my head

I can knock my head on the wall as hard as I can.

I was really thinking real hard for solution.
But I got nothing out of it.
It's been 1hrs and i'm still on that question.
When I thought I have get the answer for part 1 and wanted to go to part 2 then I find out that i'm wrong. Trying to just do part 2 first but end up getting the same problem. Looking back at my answer again and again will only just let me knock my head again and again.
Dateline is tomorrow and i'm still on the same spot. Haiz.....