A classic riddle is:
 
Q: How many sides does a ball have?
A: Two—the inside and the outside.
 
“How many sides has a tree?” was a question asked in many newspapers in 1851. “How many sides has a circle?” was printed in Harper’s Bazar (New York, NY) in 1875. “How many sides does a barrel have?” was printed in many newspapers in 1964.
   
“How many sides does a ball have? 2 inside & outside” was posted on Twitter on April 8, 2010.
 
         
23 July 1851, Hartford (CT) Daily Courant, pg. 2, col. 2:
How many sides has a tree?
Two. The inside and outside.
 
11 December 1875, Harper’s Bazar (New York, NY), “Facetiae,” pg. 808, col. 2:
“How many sides has a circle?”
   
“Two,” said the student.
   
“What are they?”
   
What a laugh in the class the student’s answer produced when he said, “An inside and an outside!”
 
Google Books
April 1876, The Australian Journal, pg. 464, col. 1:
“How many sides has a circle?”
 
“Two,” said the student.
 
“What are they?”
 
What a laugh in the class the student’s answer produced when he said, “An inside and an outside!”
 
16 November 1964, Sioux Falls (SD) Argus-Leader, “Fun Time—The Riddle Box” by A. Leokum,  pg. 7, cols. 7-8:
How many sides does a barrel have?
The inside and the outside.
         
Google Books
Highlights Book of Science Questions that Children Ask
By Jack Myers
New York, NY: Barnes & Noble Books
1995
Pg. 244:
How many sides does a ball have? If we should decide that it has only one, then we would say that the whole earth has only one side. That is, unless you think about the idea of inside and outside.
 
The Math Forum
Does a Sphere have Sides?
Date: 04/20/2001 at 10:18:28
From: Doctor Peterson
Subject: Re: Sides of a sphere
 
Hi, Sarah and Stephany.
 
I’d say it’s a definition question: what do you mean by “side”?
(...)
I think the most applicable definition is the one used in topology. We talk about surfaces with two sides (like a piece of paper) and surfaces with only one side (like the Moebius strip you may have heard of). In this sense, and remembering that mathematicians define a sphere as the surface of a sphere, not a solid ball, I would say that a sphere has two sides: the outside and the inside. That may sound like a joke, but it’s really the best mathematical answer I can give.
     
Twitter
Best jokes‏
@Hultz1
how many sides does a ball have? 2 inside & outside.
6:19 PM - 8 Apr 2010
 
Google Books
Trans-incidental American Adventures via the Railways
By Erik “Skippy” Sund
Published by author
2014
Pg. 93:
He asked “How many sides does a ball have?” I answered 2. He asked “Why did you say that? What are the sides?” I replied the inside and the outside.
 
Reddit—Jokes
How many sides does a ball have?
submitted August 7, 2017 by TheHappinessSeeker
Two. The inside. And the outside.