An analog clock has “hands,” but a digital clock doesn’t. A riddle has been cited in print since at least 1974:
 
Q: What did the digital clock say to its mother?
A: Look, ma! No hands!
 
A grandfather clock version has been cited in print since at least 2004:
 
Q: What did the digital clock say to the grandfather clock?
A: Look, grandpa! No hands!
 
“What did the digital clock say to the analog clock?”/“Look, no hands!” is a similar joke.
 
     
Google Books
April 1974, Boys’ Life, “Think and Grin,” pg. 74, col. 2:
Giuseppe: What did the digital clock say to its mother?
Leonardo: I know — look, Ma, no hands! — James Cunningham, Oak Lawn, Ill.
 
24 November 1974, The Robesonian (Lumberton, NC), “Mini Jokes,” The Mini Page, pg. 3, col. 1:
Q. What did the digital clock say to its mom?
A: Look mom! No hands!
Anthony Dill of Lumberton, age 9
         
28 December 1974, Battle Creek (MI) Enquirer and News, “Mini Jokes,” The Mini Page, pg. 3, col. 1:
Q. What did the baby digital clock say to the n=mother digital clock?
A. Look, mom, no hands!
—Gennel Hemminger, Battle Creek
 
11 January 2004, Newsday (Long Island, NY), “Smiles,” Kidsday sec., pg. 1:
What did the digital clock say to the grandfather clock?
“Look, Grandpa, no hands!”
 
Google Books
A Chuckle Of Jokes
By S.M. Mathur
New Delhi, India: Param Offsetters
2009
Pg. ?:
Q. What did the digital clock say to the grandfather clock?
A. Look grandpa, no hands.
 
Graphic Jokes
What did the digital clock say…
May 8, 2012 by golly
Q: What did the digital clock say to the grandfather clock?
A: Look grandpa! No hands!
     
Reddit—Jokes
What did the young digital clock say to its Grandfather clock?
submitted March 31, 2013 by Gabeito
“Look Grandpa, no hands!”
 
Twitter
Ellen DeGeneres‏
@TheEllenShow
What did the the digital clock say to the grandfather clock? Look grandpa! No hands! #ClassicJokeWednesday
4:31 PM - 12 Feb 2014
     
Google Books
The Super, Epic, Mega Joke Book for Kids
By Whee Winn
Grand Rapids, MI: Zonderkidz
2016
Pg. ?:
What did the digital clock say to its mother?
Look, Mom, no hands!
 
Reddit—Clean Jokes
What did the digital clock say to the grandfather clock? (self.cleanjokes)
submitted November 19, 2016 by madazzahatterQuality “Ugh” Contributor
“Look grandpa, no hands!”