A horse walks into a bar. The bartender asks the horse if they're an alcoholic considering all the bars they frequents, to which the horse replies "I think not!". POOF! The horse disappears.

This is the point in time when all the philosophy students in the audience begin to giggle, as they are familiar with the philosophical proposition of "cogito ergo sum", or "I think, therefore, I am".

But to explain the concept beforehand would be putting Descartes before the horse.