Realschule Puns

Background: I'm living in Germany and have a German roommate who can speak some English. He did Realschule (completed 10th grade), but mostly ditched classes. I try to teach him when I can and vice versa.

We were watching Walking Dead. The episode was right after the prison gets stormed. Rick and Carl lock themselves in a house, and Carl ties a knot that he's super proud of... Blablabla... A few minutes later, knot is quickly falling apart as zombies try to break in.

I absentmindedly said: "Cool knot, bro" Roommate: "(K)not cool, bro"

I just looked at him with a mixture of surprise, pride, and wondering still if he knows what a dad joke is.