“Why did the boy eat his homework?”/“Because his teacher told him it was a piece of cake.”

Something that is a “piece of cake” is something that is easy or simple. There’s a school joke:
 
Q: Why did the boy eat his homework?
A: Because his teacher told him it was a piece of cake.
 
The joke has been cited in print since at least 1993.
 
   
Google News Archive
24 May 1993, Beaver County Times (Beaver, PA), “Class Acts,” pg. C2, col. 1:
Q: Why did the kid eat his homework?
A: Because his friend told him ti was a piece of cake.
 
28 January 1998, Los Angeles (CA) Times, “Laugh Lines,” pg. E3, col. 3:
Why did the student eat his homework? The teacher told him it was a piece of cake.
(Saul Castaneda, 9, Redondo Beach, Lincoln Elementary)
 
Google Books
Over 1500 Fantastic Jokes
By Guy Campbell and Paul Moran
London: Dean
2002
Pg. 120:
Why did the student eat his homework?
The teacher told him it was a piece of cake!
 
Twitter
Edgar Garcia
‏@egspoony
@exrabutler Why did the student eat his homework? The teacher told him it was a piece of cake.
10:38 AM - 10 Oct 2008
 
Twitter
Chris Happel
‏@HappelCore
Why did the student eat his homework?..........Because his teacher said it was a piece of cake….oh hahaha
3:16 PM - 4 Apr 2009
 
Google Books
The Mammoth Book of Really Silly Jokes
By Geoff Tibballs
London: Constable & Robinson Ltd
2011
Pg. 120:
Why did the boy eat his homework?
Because his teacher said it was a piece of cake.
 
YouTube
Jokes - Why did the boy eat his homework? His teacher said it was a piece of cake.
jokesus
Published on Oct 5, 2015
Joke :
Why did the boy eat his homework?
His teacher said it was a piece of cake.
 
Google Books
The Super, Epic, Mega Joke Book for Kids
By Whee Winn
Grand Rapids, MI: Zonderkidz
2016
Pg. ?:
Why did the student eat his homework?
The teacher told him it was a piece of cake.
 
reddit
Why did the boy eat his homework?
submitted August 21, 2016 by Kenzlea
Because his teacher told him it was a piece of cake.