build a fire under sb：鼓励；刺激，激励

e.g. The teacher built a fire under the students, and they really started working.

老师给学生们做了一番鼓励，他们果真开始努力学习了。

e.g. Somebody built a fire under Bill, so he finally went out and got a job.

比尔受到鼓励，最终出去找到了工作。