A riddle about sardines is:
 
Q: Why are sardines so stupid?
A: They lock themselves in a can and leave the key outside.
 
The joke has been cited in print since at least 1959.
 
     
1 February 1959, South China Sunday Post-Herald (Hong Kong), “Heard these?,” pg. 17, col. 5:
Q. Why are sardines such silly fish?
A. Because they hide themselves in little tins and leave the key outside.
     
Google Books
Smokey the Shark, and other fishy tales
By Charles Keller
Illustrated by Lee Lorenz
Englewood Cliffs, NJ: Prentice-Hall
1981
Pg. 20:
Why are sardines such silly fish?
Because they crawl into a can, lock themselves in, and leave the key outside.
 
23 April 1989, The Sun (Baltimore, MD), The Kids’ Pages, pg. 1, col. 2:
Q: Why are sardines such silly fish?
A: Because they crawl into a can, lock themselves in, and leave the key outside.
   
Google Books
Jokes and Humor for Children
By Thomas Philamer I Andres
Manila, Philippines: Our Lady of Manaoag Publishers
1990
Pg. 168:
Question: Why are sardines such silly fish?
Answer: Because they crawl into a can, lock themselves in and leave the keys outside
       
Twitter
Fish Chat
@Fish_chat
Why are Sardines the stupidest fish ever?...They climb into tins - Close the Lid - And then leave the Key on the OUTSIDE!
7:34 PM - 30 May 2009
 
Google Books
Sensible Nonsense.. Frank’s Comic Pack:
Humour Pack Series
By Frank Paul
BookBaby
2014
Pg. ?:
“Why are sardines the stupidest fish in the sea ? Because they climb into tins, close the lid and leave teh key outside !”
 
Twitter
Hobo Nickel Society
@Hobo_Nickel_Soc
Q: Why are sardines so stupid?
A:  They lock themselves in a can and leave the keys outside
#HoboNickel #CoinCarving by #PaoloCurcio @elmrthe
#TellAnOldJokeDay #NationalTellAnOldJokeDay
3:09 PM - 24 Jul 2018