For instance, early in your first lecture in the relativity unit, you flash the following screen on a slide.
You can poll the students using Clickers, or use software such as Poll Everywhere that lets them answer on their phones, or just have them hold up sheets of paper with letters on them. If a number of students get the wrong answer, you might give them 1-3 minutes to discuss and then vote again before showing them the correct answer.
The right answer is always on the next slide.
Many studies show that if you insert 2 or 3 such quick breaks into a 50-minute lecture, student understanding and retention improve significantly. Another advantage, if you use the clickers, is that you sometimes discover that the class is really not with you. That's unfortunate, but it's better to know!
The files below contain two types of clicker questions.