“If swimming is such good exercise, explain whales (to me)” is a joke on many websites that has been printed on many gift items, such as T-shirts and posters. “If swimming is so good for the figure, how do you explain the whale?” has been cited in print since at least September 1973, when newspapers began printing the line as “Today’s Chuckle.”
 
“Pandas are living proof that you can be fat just by eating salad” is a similar joke about another seemingly fat animal.
 
   
24 September 1973, The Oregonian (Portland, OR), pg. 1, col. 5:
Today’s chuckle
If swimming is so good for the figure, how do you explain the whale?
 
6 July 1975, Chicago (IL) Tribune, “Allyn wrong for blaming press for Sox problems” by Cooper Rollow, pg. B3:
Joe McGuff’s thought for the day: “If swimming is so good for the figure, how do you explain the whale!”
   
Google News Archive
24 March 1977, The Spokesman-Review (Spokane, WA), pg. 14, col. 8 ad:
If swimming is so good for the figure, how do you explain the whale?
(“Shop with Sue” column—ed.)
 
Google News Archive
8 January 1995, The Daily Sentinel (Pomeroy-Middleport, OH), pg. 4, col. 6 ad:
If swimming is so good for the figure, how do you explain whales?
(Rutland Furniture—ed.)
 
15 April 1999, Kansas City (MO) Star, “Here’s something to make April 15 a little less taxing” by Lewis W. Diuguid, pg. 11:
If swimming is so good for your figure, how do you explain whales?
 
Google Books
The Good Sports Joke Book
By Paul M. Miller
Uhrichsville, OH: Barbour Publishing
2004
Pg. 64:
NEED ANSWER
If swimming is so good for your figure, how do you explain whales?