Here's my personal favorite:

You know Rohto V eye drops? Well the first time you use them they burn like crazy. Most people who use them for the first time go "wtf, why are these burning so bad". So what you do is while they're complaining about the burning you look them straight in their good eye and go "You ever tried Acid before?....because you just did." Most people FLIP out depending on how high they are. I had this done to me the first time I used Rohtos and I have done it to many people since. Best shit ever. If you've never done it then it is a must do.