How do you sink a Norwegian submarine?
Scuba-dive down and knock on the door.

How do you sink a Norwegian submarine again?
Dive down and knock on the door again. Wait for them to open the window and say, "You aren't fooling us this time!"

How do you sink a Danish submarine?
Dive down and knock on the window. Wait for them to open the door and say, "Come on, who do you take us for? Norwegians?"

How do you sink a Swedish submarine?
Give it a Norwegian crew.

lel. I love Norwegian humor.