A popular joke about boxes that say “shake well” is:
 
Q: Why did Johnny jump up and down before drinking his juice?
A: Because the box said, ‘Shake well.’”
 
Forms of the riddle/joke have been cited in print since at least 1933, when the “shake well” involved medicine. The early jokes had the person read the instructions too late, and then start jumping up and down. “Q: Why did Johnny jump up and down before he drank his juice? A: The carton said to ‘shake well before drinking’”—the juice version—was cited in a 2010 joke book.
 
“This orange juice says shake well before drinking…” (drinking alcohol) is another “shake well” joke. “Why did the little kid dance on the jar of jam?”/“Because the top said, ‘Twist to open’” is a “twist to open” joke.
       
 
31 May 1933, Kingston (NY) Daily Freeman, “Office Cat” by Junius, pg. 6, col. 1:
A local man, unable to sleep, took a couple of “sleeping tablets” then noticed that they should have been dissolved in hot water. So he hurried to the faucet and gulped down a glassful of tepid water. His act is reminiscent of the small boy, who, spied the “shake well before taking” label and began jumping up and down. 
 
1 March 1964, Leavenworth (KS) Times, “Jokes,” The Junior Times, pg. 19, col. 3:
Toby: “Why are you jumping up and down?”
Tony: “I just took some medicine and the instructions were to shake well, and I forgot it.”
Janet Gillespie
No address given
 
28 January 1966, The Daily Herald (Provo, UT), “Tell Me Why” by A. Leokum, pg. 8, col. 8:
Sally: Why are you jumping up and down?
Willie: I just took some medicine and it says: shake well.
   
Google Books
The Silly Kid Joke Book
By Caroline Anne Levine
New York, NY: Dutton
1983
Pg. ?:
“Why did the silly kid jump up and down before taking his medicine?
Because the bottle said Shake Well Before Using.   
 
22 September 1983, Lexington (KY) Herald-Leader, pg. D6, col. 4:
Why did the man jump up and down before taking his medicine?
The label read: “Shake well before using.”
By Carolyn Stidham
Grade 6
Georgetown Middle School
 
Google Books
Super Sick Jokes and Riddles
By Joseph Rosenbloom
Pictures by Sanford Hoffman
New York, NY: Sterling Publishing Company
1990
Pg. 36:
Why did the silly kid jump up and down?
Because the medicine label said, “Shake well”
 
Google Books
Laugh-Out-Loud Jokes for Kids
By Rob Elliott
Grand Rapids, MI: Revell (Baker Publishing Group)
2010
Pg. 10:
Q: Why did Johnny jump up and down before he drank his juice?
A: The carton said to “shake well before drinking.”
 
Twitter
Gil Arellano
‏@gilbertoid
Q. Why did the blonde jump and dance before drinking some orange juice? A. Cause it said shake well!.
8:09 AM - 27 May 2010
 
Google Groups: Clean Humor
Jumping Up and Down
KB
10/15/12
(Thanks to Good, Clean Funnies for today’s humor.  KB)

A pharmacist looks out the front of the store and sees a woman holding a bottle while jumping up and down in the parking lot. The pharmacist walks out to the parking lot and asks the woman, “What’s the matter?”
 
She replies, “I didn’t notice until after I took the medicine that it said, ‘Shake Well.’”
   
Explain the Joke
Shake Well
Posted on April 12, 2013 by stfleming
Q: Why did John jump up and down before drinking his juice? juice box
A: The box said, “Shake well before drinking!”
 
Explanation:  Johnny was sooooooo confused!  When the box says, “Shake well before drinking” it is talking about the box, not the boy who is drinking the juice!  Shake the box, Johnny!
 
Twitter
Carol Anne
‏@cacisa2j
JOKE:
-Why did Johnny jump up & down before he drank his juice?
-Because the carton said “Shake well before drinking.”
7:57 PM - 16 Sep 2016