Nobody is going to get this joke

Today was this semester's organizational meeting of the student combinatorics seminar, when we decide on what topics we want to hear about and try to see who we can get to talk about them. One topic last year that we agreed on was the Artic Circle theorem; however, we ultimately didn't end up getting anyone to talk about it so that never got done. So this semester, the Arctic Circle theorem was first on the list; we didn't even count votes for it, Chris just drew a box around it to indicate that it's in. Later, when circling the other topics we'd agreed on, the ones with lots of votes, he drew circles around them.

Later, someone who'd missed this asked, "Why does the Arctic Circle theorem have a box around it?"
Someone replied: "Because it's a frozen vertex."