From Wikipedia

The US Postal Service released a stamp featuring Ogden Nash and six of his poems on the centennial of his birth on 19 August 2002. The six poems are "The Turtle," "The Cow," "Crossing The Border," "The Kitten," "The Camel" and "Limerick One." The stamp is also the first stamp in the history of the USPS to include the word "sex," though as a synonym for gender, not as the act. It can be found under the "O" and is part of "The Turtle".