This one is one of my favorites from the Martin Gardner collection:

My wife and I recently attended a party at which there were four other married couples. Various handshakes took place. No one shook hands with himself or herself or with his or her spouse, and no one shook hands with the same person more than once. After all the handshakes were over, I asked each person, including my wife, how many hands he or she had shaken. To my surprise each gave a different answer. How many hands did my wife shake? 

Happy hand and mind shaking!