A horse walks into a bar. 

The bartender says, "You're here a lot. Are you an alcoholic?" 

The horse ponders for a minute and responds, "I don't think I am." 

And "poof", he disappears. 

This is where philosophy students start to snicker, as they are familiar with Descartes postulate, "I think, therefore I am." 

But telling you that first, would be putting Descartes before the horse.