The professor paused, and the long-suffering student knew instinctively that he was in for another performance of the infamous and infinitely annoying joint-cracking ritual. It began, as it always did, with the knuckles... an innocent enough action in and of itself. The professor, however, felt compelled to pop every joint in every finger... one at a time.
"The common grackle", he said, cracking the left and right index fingers in quick succession, "is not a bird to be taken lightly. No, I dare say... not at all." His neck came next, bending in a number of seemingly deadly directions, especially for a man of the professor's great age. Not that anyone was quite sure just how old the professor was.
"It's technically just an overgrown blackbird, but with startlingly unique behavior patterns. Irregular is one way of describing it, I suppose." With that, the aged academic dipped a bit, simultaneously cracking the joints in both knees. The student was disgusted and yet secretly impressed.
"Irregular?" the student asked when he had regained his composure, hoping that the interruption would somehow cause the old man to forget the what he was doing.
"Precisely" he said, while quickly popping his right elbow with sheer force of will.
"They're scavengers through and through, and thus live in a constant state of starvation, barely able to gather the nutrients they need to survive. And yet despite their need to conserve whatever energy they can manage to scrape off of the sidewalks, they waste a prodigious amount of time playing games, taunting other creatures for no discernable reason, and in general being a nuisance to all of nature. Their recent behavior is only building upon already well-established personality quirks."
Through the course of his proclamations, the old man had managed to crack his other elbow, his jaw, each toe, and somewhat incredibly, his ears. The student could only pray that his performance had come to an end.
"But they're playing hopscotch!" the pupil said. "They even draw the board themselves, by holding rocks in their beak."
"GAME PLAYING!" the professor said, popping his hip for emphasis. "As I said... well documented behavior. Grackles have long been known to play a form of tag, especially when competing for the affections of the opposite sex."
With that he cracked nearly every vertebrae in his spine, and the ghastly noise that issued forth sounded for all the world like a little kid had just gotten a hold of some bubblewrap.
"Yes", said the student, "but they didn't keep score".