It's fun to see look of shock on a persons face when they realize what happened and what they felt.

Another exercise is to have a student stand with his back to you, then walk slowly (and silently) forward. The student is to raise a hand when they feel you coming up behind them.

You can vary this by thinking angry thoughts and happy thoughts and watch distance at which you become 'noticed' by student.

The purpose of teaching a student to feel intention is to empower them in situations that most people find daunting. When you KNOW that another person has ill intentions towards you, then you are in control. Depending on situation you can leave, you can seek a diplomatic solution or you can wait till future attackers back is turned and hit him with a chair.

While some of these solutions are better than others (Chair Fu should always be absolute last resort) what's important here is that because of your heightened awareness you weren't taken by surprise when someone attacked you.

For an interesting illustration of using intention and feeling difference between intent to strike and actual strike read Chapter 6 of Dr. Glenn Morris' book "Pathnotes of an American Ninja Master".