“When is soup musical?”/“When it’s piping hot.”

A popular soup joke is:
 
Q: When is soup musical?
A: When it’s served piping hot!
 
“When is water musical?” was cited in print in 1995 and “When is a plum pudding musical” was cited in 1997. “When is soup musical?” has been cited in print since at least 2010. Campbell’s Chunky Soups used the soup joke on Twitter in 2013.
 
 
Google Books
Just for Fun
By Betty Root
Aylesbury: Ginn
1995
Pg. 61:
“When is water musical?” he asked.
 
The Jester held up his pipe. “When it’s piping hot!”
 
Google Groups: rec.humor
More crackers
David Bridges
6/25/97
(...)
When is a plum pudding musical?
When it’s piping hot.
 
The Cheapest Man In America
Friday, September 3, 2010
i>Got a few jokes for you before supper tonight…
Q-What do you call a chicken in a hottub?  A-  Soup!!!
Q-Who is the head of the soup factory?      A-Souperman!
Q-When is soup musical?                  A-When it’s piping hot!
 
Mosaic Kitchen
November 11, 2011
A Thick and Delicious Homemade Soup in 15 Minutes—No Joke!
Posted by Janet DeGras
Q: “When is soup musical?”
A: “When it’s piping hot!”
~ a very old joke of unknown origin
 
Google Books
The Mammoth Book of One-Liners
By Geoff Tibballs
London: Constable & Robinson Ltd.
2012
Pg. ?:
When is soup musical?
When it’s piping hot.
 
Twitter
Campbell’s Chunky
‏@CampbellsChunky
Q: When is soup musical? A: When it’s piping hot. pic.twitter.com/Rri04owlaa
9:30 AM - 18 Nov 2013