In Emily Riehl's Topology I: Topological Spaces and the Fundamental Group, she uses a fun heads-up-seven-up style quiz to quickly engage students and test the level of confusion among the students. This activity not only helps students participate and actively take a part in their learning, but also assists the instructor in increasing the effectiveness of their lectures.
Goals: The goal of this activity is to test whether students understood definitions and concepts given previously in the lectures and also in the readings. It should make the students think for themselves and to give the instructor real-time feedback about the class' level of confusion.
Procedure: At various points throughout the semester, I'd pause the lecture, to ask a true/false question meant to test understanding of a particular concept or definition. Occasionally the questions were easy, but more often they were of moderate difficulty. Then I'd ask them to put their heads down on their desks and close their eyes "heads-up-seven-up" style. They'd answer by raising their hands to vote. I could quickly gauge the degree of misunderstanding without embarrassing anyone. Then we'd talk about it as a class. The first time I did this I made a big deal of referring to the "clickers" used in the other sciences. Mathematicians pride ourselves on being contentedly low-tech.
Comments: This kind of activity takes minimal time and effort and can make lectures much more effective. The instructor recommends that one must present enough confidence to get the students to buy into a silly activity. But they will.