“A guy walks into a bar…” is a typical form of what has been called the “bar joke.” A popular science joke that has been printed on many images is:
 
Two men walk into a bar.
One man orders H2O.
The other says, “I’ll have H2O too.”
The second man dies.
 
The second man was given hydrogen peroxide (H2O2). The joke has been cited in print since at least 2005, when it was posted on Slashdot.
   
Another version of the joke (with an altered punchline) has been cited in print since at least 2012:
 
Two chemists walk into a bar.
The first one says, “I’ll have some H2O.”
The second says, “I’ll have some water too. But why’d you order it like that? We aren’t at work.”
The first chemist excuses himself and weeps in the bathroom.
His assassination plot had failed.
         
 
Yahoo! Answers (August 10, 2012)
I don’t get it…joke?
Two chemists walk into a bar.
 
The first one says, “I’ll have some H2O.”
 
The second says, “I’ll have some water too. But why’d you order it like that? We aren’t at work.”
 
The first chemist excuses himself and weeps in the bathroom. His assassination plot had failed.
 
Google+
Joke of the Day
Public
Oct 09, 2012
Two chemists walk into a bar.
The first one says, “I’ll have some H2O.”
The second says, “I’ll have some water too. But why’d you order it like that? We aren’t at work.”
The first chemist excuses himself and weeps in the bathroom.
His assassination plot had failed.
     
Quora
What are some famous chemistry jokes/memes?
Sudhindra Sajjal, Confused Undergrad
Written 7 May 2014
(...)
Two chemists walk into a bar,
The first one says, “I’ll have some H2O.”
The second says, “I’ll have some water too. But why’d you order it like that? We aren’t at work.”
The first chemist excuses himself and weeps in the bathroom.
His assassination plot had failed.
 
STEAM Community
Megadog14  has PAYDAY 2 Jul 8, 2014 @ 7:26pm
Two chemists walk into a bar.
 
The first one says, “I’ll have some H2O.”
 
The second says, “I’ll have some water too. But why’d you order it like that? We aren’t at work.”
 
The first chemist excuses himself and weeps in the bathroom.
His assassination plot had failed.
   
Reddit—Jokes
Two scientists walk into a bar The first one says “I’ll have some H2O.” The second one says, “I’ll have some H2O too.”
submitted December 12, 2014 by thefran
The bartender pours them both hydrogen peroxide because he’s tired of their bullshit every day
(...)
COMMENTS
Not_a_spambot
My favourite variant on this joke:
Two chemists walk into a bar. The first says “I’ll have some H2O.” The second says “I’ll have some water as well, but why’d you order it that way? We aren’t at work anymore.”
The first chemist excused himself and went to weep quietly in the bathroom. His assassination plot had failed.
 
Reddit—Jokes
Two chemists walk into a bar.
submitted August 10, 2016 by Booey123
The first one says, “I’ll have some H2O.” The second says, “I’ll have some water too. But why’d you order it like that? We aren’t at work.” The first chemist excuses himself and weeps in the bathroom. His assassination plot had failed.
     
Reddit—Jokes
Two chemists walk into a bar.
submitted February 14, 2017 by CesarChace345
Two chemists walk into a bar.
The first one says, “I’ll have some H2O.”
The second says, “I’ll have some water too. But why’d you order it like that? We aren’t at work.”
The first chemist excuses himself and weeps in the bathroom.
His assassination plot had failed.