Wear a Bikini!
From Homestar Runner Wiki
"Wear a Bikini!" is the slogan towed behind planes and ships on certain Main Pages, and the Strong Bad Email island. As such, it is an answer to Question Three of the Homestar Quiz. In Main Page 23, it appears on the back of Strong Sad's "floating head of doom". Its larger significance is lost on most, as any deeper meaning to this directive to don a two-piece bathing suit has not been revealed by The Brothers Chaps. A variation on this joke is seen in looking old where a plane with a banner carries the slogan "Wear a Diaper!".