I wish this were a metaphor, or something deep, but it’s not. It’s just cute.
Last week, one of my multivariable calc students was late to class. No excuse, he just forgot to leave the break period on time. He was a good 5 minutes late, so me and the other students closed and locked the door, and when we heard him knocking, we starting talking — really loudly — about all this candy we had that we were eating, and throwing around calculus terms. It didn’t make sense, what we were saying. It was just us having a little fun.
After 30 seconds of this, we let him in, and I was like: “Oh, guys, we’re so silly. We should have moved the whiteboard (it’s on wheels in this classroom) in front of the door.”
Of course, Monday comes about, and I open the door to my classroom, and what do I see and hear? I see a whiteboard covering the entrance, and I hear my students — who have all arrived early to do this — talking loudly about candy.
It warmed the cockles of my heart. (Not that I know what a cockle is, nor whether my heart has them or not. But still. You get the point.) 
 Okay, I had to look this up. This is what I found.
Hmm, the OED agrees with the last suggestion offered by the linked article. But incidentally I happened to find this other fun thing while poking around the dictionary:
“hot cockles, n: A game in which one player lies face downwards, or kneels down with the eyes covered, and attempts to guess which of the other players has struck him or her on the back.” (http://dictionary.oed.com/cgi/entry/50108438)
Be glad your students weren’t playing at that.