Riddles, Puzzles, Brain Teasers, Oh My!

I open each psychology class with a riddle/puzzle/brain teaser/etc. I am finding it difficult, however, to find good ones! I thought it would be very easy, but it turns out that is not the case. Therefore, I need your help! Do you have a favorite brain teaser? A puzzle that always makes people think? A riddle with a clever answer? Let's hear'em in comments.

Teaching Psychology

I teach a course on psychology and know there are nearly limitless topics we could pursue, but also know that I cannot think of them all. That is where you all come in. Did you take psychology? Are you a psychology guru? Do you simply have a good idea? Put your "I'm a student" shoes on and answer this question: What topics do you think would be awesome?

Here are some ideas to get your brain warmed up:
*Male-Female interaction
*Phobias and Anxieties
*Addiction
*Love

The floor is yours...