“Why did the tomato turn red?”/“Because it saw the salad dressing.”

Q: Why did the lobster blush?
A: Because he saw the salad dressing!
 
“Lobster blush” jokes date to at least 1876; “the lobster blushed because he saw the salad dressing” jokes date to 1901. In modern variations of the joke, the lobster’s blushing has been replaced by “Why did the tomato blush/turn red?”
 
“Why did the tomato blush?”/“Because it saw the salad undressing” (with “undressing” instead of “dressing”) is a related riddle. “Why did the cranberries turn red?”/“Because they saw the turkey dressing” is a popular Thanksgiving/Christmas version of the riddle.
 
   
11 January 1876, Inter Ocean (Chicago, IL), pg. 4:
Why does the shy lobster blush so after being boiled? Because he’s gone to pot. Te, he he!
 
9 December 1891, Omaha (NE) World-Herald, pg. 4:
WEIRD ALTRUISTIC PHILOSOPHY.
Boston Post: The principal difference between a lobster and a lobbyist is that you can make a lobster blush.
   
Newspapers.com
20 March 1901, Atchison (KS) Daily Globe, pg. 5, col. 3:
New minstrel joke: (...) Another: When did the lettuce blush? When it saw the salad dressing.
   
Newspapers.com
20 April 1901, Macon (GA) News, pg. 1, col. 2:
On a Vineville car the following took place between two of Macon’s most charming society young ladies: “I say, Lil, can you tell me when did the salad blush.” Lil had to give it up, and in a stage whisper she was informed, “when it saw the salad dressing.”
 
Newspapers.com
9 May 1901, The Cambria Daily Leader (Swansea, Wales), pg. 2, col. 3:
SEEN AND HEARD
When did the lobster blush?—When he saw the salad dressing.—“Punch.”
 
4 July 1901, Bucks County Gazette (Bristol, PA), pg. 3, col. 1:
When did the lobster blush? When he saw the salad dressing.
 
17 July 1901, Waterloo (IA) Daily Courier, pg. 3, col. 3:
Why did the lobster blush? Because he saw the salad dressing.
 
20 December 1901, Kalamazoo (MI) Gazette, pg. 8:
Why did the lobster blush? Because he saw the salad dressing. And then they went down to Fuller’s, 228 east Main street, to buy their engagement ring.
 
Newspapers.com
8 January 1903, Enid (OK) Weekly Wave, “Copyrighted Chestnuts,” pg. 8, col. 5:
Why did the lobster blush?
When it saw the salad dressing.
 
Google Books
Rips from the Buzz Saw
By Albert U. Mayfield
Published by the author
1903
Pg. 6:
The lettuce that blushed so coquettishly when it saw the salad dressing, completely wilted when it found that the hot grease, had soured on it.
     
Papers Past
7 March 1903, The New Zealand Observer (New Zealand), pg. 16:
The “What made the lobster blush, Because it saw the salad dressing” variety of inanity is giving way to this sort of thing.
     
Google Books
February (?) 1906, Meyer Brothers Druggist (St. Louis, MO), pg. 18, col. 2:
The Lobster Turned Red when he saw the salad dressing.
   
16 September 1906, Fort Worth (TX) Star-Telegram, second section, pg. 4:
There’s a scandal in our kitchen—
Indeed, it’s most distressing—
A man peeped through the transom
And saw the salad dressing.
 
Newspapers.com
18 January 1917, The Cedar County News (Hartington, NE), pg. 2, col. 4:
THINGS TO WORRY ABOUT
Did the tomato bush blush because it saw the salad dressing?
 
Google News Archive
1 June 1921, Sydney (Australia) Mail, pg. 21, col. 2:
Why did the lobster blush?—Because he saw the salad dressing.
     
Google Books
Crewe Train
By Rose Macaulay
London: W. Collins Sons & Co.
1926
Pg. 153:
A woodpecker, that’s a woodpecker, because the woodpecker would peck her, why did the lobster blush, because it saw the salad dressing,...
   
Newspapers.com
4 July 1927, Wilmington (DE) Morning News, “Fruit-Vegetables Used Doubling,” pg. 9, col. 7:
It is historically asserted that the tomato turned red because it saw the salad dressing, but its nearest friend, the lettuce, has no cause to blush.
 
Google News Archive
9 July 1927, St. Petersburg (FL) Evening Independent, “Eating More Vegetables and Fruit,” pg. 2, col. 2:
It is historically asserted that the tomato turned red because it saw the salad dressing, but its nearest friend, the lettuce, has no cause to blush.
     
Google Books
September 1928, The American Chamber of Commerce Journal, pg. 13:
In The Birthday Cake, Laurel and Hardy depend upon the time-honored bon mot about the lobster having blushed because he saw the salad dressing. This witticism has been perverted to undressed salad,...
     
Newspapers.com
5 July 1930, Vancouver (BC) Sun, “Riddles,” sec. 3, pg. 7, col. 6:
“What made the tomato blush?”
“When he saw the salad dressing.”
 
Newspapers.com
26 August 1938, Windsor (ON) Daily Star, “The Roving Reporter” by Thomas R. Brophy, pg. 16, col. 4:
The proceedings may even be livened up by trick questions such as, “When did the tomato turn red?”
 
The answer, in case you’ve forgotten, is, “When it saw the salad dressing.”
 
Google Books
The American Riddle Book
By Carl Withers, Sula Benet and Marc Simont
New York, NY: Abelard-Schuman
1954
Pg. 77:
Why did the lobster blush?
—Because it saw the salad dressing.
       
Google Books
AVKO’s All New Word Families in Sentence Context
By Don McCabe
Clio, MI: AVKO Educational Research Foundation
1996
Pg. 157:
Did the tomato really blush when it saw the salad dressing?
       
Newspapers.com
20 August 1998, Thursday Dispatch (Pittston, PA), “Silly, Silly, Tomato Jokes,” pg. 27, col. 2:
Why was the tomato blushing?
Because he saw the salad dressing.
 
Google Books
Songs for my dog and other wry rhymes
By Max Fatchen
Kent Town, S. Aust.: Wakefield Press
1999
Pg. 10:
Why did the beetroot blush? They say
It saw the salad dressing.