What's the best ice breaker exercise for the class that a professor used on your first day of class?

Not an exercise, but it's the only intro to a prof I remember after 20 years: In freshman chem (in the late 90's): It was a big lecture hall with stadium seating and it was early afternoon so none of the students were 100% there.

Then this middle aged man comes jogging down the center walkway/steps with a bottle in his hand. He jogs up the the lab bench at the front of the room and pours the bottle (hydrogen peroxide in retrospect) into a large beaker and all of the sudden there was a 12+ ft column of foam shooting toward the ceiling - before most of the class even new the prof had arrived. Then he turned to us an said, "we'll learn why that happened in about 3 weeks."

He also ended every Friday lecture with a "Boom of the Week" in which he'd explode something (larger each week) in order to make sure we didn't skip Friday classes. Rumor has is it that, years before I got there, the last Friday's "Boom of Week" would involve taking the class to the river and dropping a large block of magnesium metal in the water. But the college of science had asked him to stop for fear of how it affected the fish.