A popular children’s tennis pun is:
 
Q: What can you serve but never eat?
A: A tennis ball.
 
The joke has been cited in print since at least 1968 (when the answer was a ping-pong ball).
   
 
9 March 1968, The Daily Gleaner (Kingston, Jamaica), pg. 22, col. 3: 
“Fun Time—The Riddle Box,”
What can you serve but not eat?
A ping-pong ball.
 
6 March 1973, The Daily Independent, (Kannapolis, NC), “Mini Jokes,” The Mini Page by Betty Debnam, pg. 3, col. 2:
What can you serve but cannot eat?
A tennis ball.
Angela Stanback, Kannapolis
 
23 February 1975, Washington (DC) Star-News, “Mini Jokes,” The Mini Page by Betty Debnam, pg. 3, col. 1:
What can you serve but never eat?
A tennis ball.
 
Google Books
Biggest Riddle Book in the World
By Joseph Rosenbloom
New York, NY: Sterling Publishing Company, Inc.
1976
Pg. 96:
What can you serve but never eat?
A tennis ball.
 
Google News Archive
17 March 1976, The Free Lance-Star (Fredericksburg, VA), “Fun Time—The Riddle Box.” pg 25, col. 2:
What can you serve but not eat?
A tennis ball.
 
Google News Archive
7 June 1978, Spokane (WA) Daily Chronicle, “Kids-Only Club” by Shari Lewis, pg. 71, col. 2:
YESTERDAY’S BRAIN TWISTER: What can you serve but can’t eat? ANSWER: A tennis ball.
 
Google Books
101 American English Riddles:
Understanding Language and Culture Through Humor
By Harry Collis
Lincolnwood, IL: Passport Books
1996
Pg. 38:
What can you serve, but never eat?
A tennis ball.
To serve means “to put food in front of someone.” Serve is also a term used in the game of tennis. It means “to start the game by hitting the ball to one’s opponent.”
 
Google News Archive
2 April 1997, Rome (GA) News-Tribune, “A Little Bit of Humor,” The Bubble Gum Wrapper sec., pg. 1, col. 8:
Laurie: What can you serve but not eat?
Mary: A tennis ball.
 
Google Books
The Good Sports Joke Book
By Paul M. Miller
Uhrichsville, OH: Barbour Publishing
2004
Pg. 222:
What can you serve but never eat?
A tennis ball.
 
Google Groups: alt.humor.puns
Kids Puns of the Weak 06-22-04
Stan Kegel
6/22/04
(...)
What can you serve but never eat?
A tennis ball (Luis, 9)
 
Twitter
GilliganPierce
‏@GilliganPierce
What can you serve but not eat? A tennis ball!
1:34 PM - 2 Mar 2009
 
Twitter
Sofie Bieber
‏@BieberFanCrew
Q:What can you serve but never eat?
A:A tennis ball
share with your friends[:
11:56 PM - 27 Sep 2009
 
Google Books
The Mammoth Book of Really Silly Jokes
By Geoff Tibballs
London: Constable & Robinson Ltd
2011
Pg. 121:
What can you serve but not eat?
A tennis ball.