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.

In highschool we had a maths teacher who always did the birthday paradox as a kind of opener in the first lesson. https://en.wikipedia.org/wiki/Birthday_problem

He bet 10€, that there are 2 students with the same birthday in the classroom of (abt) 30 students. The class was allowed to work out, if they want to accept the bet, or not (exchanging birthdates was not allowed ofc).

Usually the students think, that it's nearly impossible, and accept the bet. Little did we know, that the probability of success was around 70% for the teacher.

I like to start a major fist-fight with the first student that dares to question my god-like authority. I deadpan an old-timey "put-up-your-dukes-sonny" kind of fisticuff.. they buy it 100% of the time. I like to let the student get a few in before I absolutely make mincemeat out of them.

After that, the students seem to really respect me. I have to maintain that bitter grudge with that one student the rest of the semester, though. At no point do I want them to think I've gone soft.

It doesn't matter what you say, just say it in a fake British accent. Make sure to awkwardly phase it out over time.

My highschool chem teacher's first words were. "Ice breakers are so boring, let's go blow stuff up!" And then imediately showed us what happened when you drop a cube of pure sodium on water.

Best fucking class I ever had.

"Good News, everyone!"

Ignoring that these exercises are generally embarrassing and uncomfortable, the "best" one I had was in a moderately sized group of students (20-25) where on the first day everyone had to find an alliteration with their first name initial and a food they liked, and introduce themselves with it. (Example: Fred + frittatas).

Virtually everyone has a food they like, yet it’s not an overly personal question, and the little alliteration helps immensely in memorizing new names and faces. I knew almost all names from this by day 2.

A friend of mine used to take all his worst reviews from Rate My Professor and read them out in class. Then he'd add "...and it's even worse than that! If you aren't prepared to deal with it you'd better drop the class." You can tell he was tenured.

Eberybody stand up. Those who can answer a question can sit down (this was the first class of the subject in the semester). Those who can't answer can either remain standing or leave the class and drop the subject. It surely broke the ice, especially since the organic chem subject was compulsory.

First year undergrad math : "Forget everything about how you did math in high school, that's not real math. Now let's start from the basics with an intro fo proofs."

Actually breaking ice

Lecturer here, and I would say it really depends on the setting and the content of the class. I tried different things that worked, pairing students to introduce each other for two minutes, then they have to present each other (small groups). Surveys with menti for large crowds, and I am also answering the questions (mix of personal/content/opinion questions to understand the general atmosphere ok the lecture hall).

In a summer school, I did something I really liked, the walking exercise. 3 rounds, 3 questions: students have to walk in the room, and when I clap they have to pair with the closest person and discuss the question. Then we share altogether. But again the content of the questions may really depend. The summer school was about transformations for sustainability, targeting PhD candidates. We were asking about an important event explaining why there were studying sustainability, one question was 'what are you good at' and the last one was 'what do you think should be transformed to become sustainable.'

I had an interesting one once:

We were paired up with the random new person next to us. Interview each other. Then, everyone gets to introduce their interview partner to the rest of the group.

Don’t. Just teach.