“How does a physicist exercise?”/“By pumping ion.”

A physics riddle is:
   
Q: How does a physicist exercise?
A: By pumping ion.
 
The pun is on the title of the 1977 bodybuilding film Pumping Iron. The joke has been cited in print since at least 1996.
 
       
Wikipedia: Pumping Iron
Pumping Iron is a 1977 docudrama about the world of professional bodybuilding, with a focus on the 1975 IFBB Mr. Universe and 1975 Mr. Olympia competitions. Directed by George Butler and Robert Fiore, it is inspired by a book of the same name by Butler and Charles Gaines, and nominally centers on the competition between Arnold Schwarzenegger and one of his primary competitors for the title of Mr. Olympia, Lou Ferrigno. The film also features segments on bodybuilders Franco Columbu and Mike Katz, in addition to appearances by Ken Waller, Ed Corney, Serge Nubret, and other famous bodybuilders of the era.
 
Wikipedia: Ion
In the physical sciences, an ion (/ˈaɪən, -ɒn/) is a particle having a non-zero net electrical charge, such as an atom or molecule whose total number of electrons is not equal to its total number of protons. A cation is a positively-charged ion, while an anion is negatively charged. Because of their opposite electric charges, cations and anions attract each other and readily form ionic compounds, such as salts.
       
Google Books
Funniest Riddle Book in the World
By Morrie Gallant    
Illustrated by Sanford Hoffman
New York, NY: Sterling Publishing Company, Inc.
1996
Pg. 10:
How does a physicist exercise?
By pumping ion.
 
2 Funny 4 U
Category: Sport jokes
Date Added: 11/10/2007
Joke: How does a physicist exercise?By pumping ion!
   
Google Books
The Mammoth Book of Really Silly Jokes:
Humour for the whole family
Edited by Geoff Tibballs
London, UK: Constable & Robinson Ltd
2011
Pg. 443:
How does a physicist exercise?
By pumping ion.
 
18 August 2011, Herald Express (Torquay, UK), pg. 57:
How does a physicist exercise? By pumping ion!
 
Google Groups: 10BitWorks
Tesla Pun
Don Smeller
6/11/13
Q: How does a physicist exercise?
A: By pumping ion!
 
Twitter
Steve‏
@SweepFan
How does a physicist exercise?
By pumping ion!
#jokes #funny #PlayOnWords
10:16 AM - 11 Feb 2017
 
Twitter
Lucky Score‏
@LuckyScoreNet
How does a physicist exercise?
By pumping
ion!
#Joke from #SportJokes in #Jokes, #Humor and #Satire
5:58 AM - 4 Nov 2017