Neighbor 1: Knock knock.
Neighbor 2: You forgot the 3rd knock.
Odin: .....
Alright, listen up, buttercup. This joke's all about wordplay with "knock." See, Odin is the Norse god, right? So instead of saying "knock, knock" like a normal person, the neighbor said "knot, knot" which sounds the same. Then they said that the neighbor forgot the 3rd "knot" which sounds like "Knut" which is a common Scandanavian name... so it has to be Odin.