Anyway, it was a huge class, in a gigantic lecture hall, but Prof. Karpf had this neat little system set up, where you could sign up to eat lunch with him. Every week, he would have lunch with a group of about 4 students, so that -- over time -- everyone who wanted to would get a chance to actually get to know him a bit. So I did this, and had a great time talking with him over lunch. Really nice guy, and extremely interesting.
At the next class meeting, I got there early (as always) and settled into my seat. Prof. Karpf showed up and began drawing a cell nucleus on the chalk board, including little diagrams of tRNA (which we were studying at the time). He was mostly facing away from us while he drew on the chalkboard, but I kept thinking, "He looks weird. I just had lunch with this guy. Why do I not recognize him?"
He was dressed the same as always, with these funny suspenders. And he has very recognizable hair, extremely curly and balding. I just kept staring at him and trying to figure out what was different. I was only about half-way back in the lecture hall, so I could see him pretty clearly. It was very weird and disconcerting.
So eventually it was time for class to start, and Prof. Karpf started lecturing, same as always. He started talking about tRNA, mostly just summarizing what we'd discussed in the previous lecture. After a couple minutes, he asked if anyone had any questions about the stuff he'd already covered. No one raised a hand. He nodded and turned around to go to his diagrams on the chalkboard, and then someone in the last row spoke up and said really loudly, "I have a question, professor."
We all turned to look, and the person in the last row stood up ... and it was Professor Karpf! WTF? He came walking down to the front of the class and explained that his identical twin brother was in town for a few days, and so he'd thought it would be funny to dress him up and bring him in to fool us.
See? Now that's the kind of prank I actually find amusing. No students were hurt in the making of this prank, but -- almost 15 years later -- I still remember every detail. I still remember the tRNA diagram on the chalkboard. I still chuckle when I remember that moment when he stood up in the back of the class. Everyone was so shocked!