A horse walks into a bar. The bartender asks the horse if it's an alcoholic considering all the bars he 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.
The Shocking Postcard
A long time ago, a wealthy man was having an affair with an Italian woman.
One night, during one of their rendezvous, she confided in him that she was pregnant.
Not wanting to ruin his reputation or his marriage, he paid her a large sum of money if she would go to Italy to have the child. If she stayed in Italy, he would also provide child support until the child turned 18.
She agreed, though she wondered how he would know when the baby was born. To keep it discrete, he told her to mail him a postcard, and write "Spaghetti" on the back. He would then arrange for child support.
One day, about 9 months later, he came home to his confused wife. "Honey," she said, "you received a very strange postcard today."
"Oh, just give it to me and I'll explain it later," he said. The wife handed the card over and watched as her husband read the card... then promptly turned white and fainted.
Alarmed, she picked up the card on the floor and read aloud: "Spaghetti, Spaghetti, Spaghetti. Two with meatballs, one without."