“How do flags talk to each other, they dont, they just wave!” is a flag riddle that was posted on Twitter on July 9, 2010. “Q: How do flags communicate? A: They wave! #dailyjoke” was posted on Twitter on June 30, 2017.
   
“How do flags greet each other on a windy day? They wave” was posted on Twitter on November 23, 2017.
   
         
Twitter
Jesse and Mike‏
@JesseAndMike
How do flags talk to each other, they dont, they just wave! LOL
8:48 AM - 9 Jul 2010
   
Archive of Our Own (2014)
These Four Walls
(...)
“How do flags say hi to each other?” Louis whispered into the darkness, glad that Liam hadn’t kicked him out.
 
“Louis, seriously? What’s going on why are you in my bed?”
 
“They don’t say hi, they just wave to each other. Isn’t that ridiculous?” Louis chuckled. “Harry told me that one.”
 
Twitter
Joan Yazze Gallegos‏
@JoanMarie87
Q: How do flags communicate?
A: They wave! #dailyjoke
12:12 PM - 30 Jun 2017
 
Twitter
thankful dad jokes‏
@dadtellsjokes
How do flags greet each other on a windy day?
They wave.
10:15 AM - 23 Nov 2017
 
Twitter
Unlimited Virals‏
@UnlimitedVirals
How do flags greet each other on a windy day?
They wave.
10:17 AM - 23 Nov 2017