A horse walks into a bar. The bartender asks the horse if it's an alcoholic, to which the horse replies, "I don't think I am." POOF! The horse disappears.
This is the point in time when any philosophy students in the audience begin to giggle, as they are familiar with the philosophical proposition of cogito ergo sum, or roughly, "I think, therefore I am."
But to explain that concept beforehand would be putting Descartes before the horse.