CALVIN: Well. I’ve decided I do believe in Santa Claus, no matter how preposterous he sounds.

HOBBES: What convinced you?

CALVIN: A simple risk analysis. I want presents. Lots of presents. Why risk not getting them over a matter of belief? Heck, I’ll believe anything they want.

HOBBES: How cynically enterprising of you.

CALVIN: It’s the spirit of Christmas.

Bill Watterson (b. 1958) American cartoonist
Calvin and Hobbes (1987-12-23)
    (Source)