Play it by ear also means to decide what to do at the last minute instead of making detailed plans.

VOA: special.2009.09.13

And then third year when I taught it which was the last time, I had 850 students in the class, making it at that point the largest course at Harvard.

到了第三年，也就是上一次开课，有850人参加，是当时哈佛人数最多的课程。

哈佛公开课 - 幸福课课程节选