“Oxygin” (oxygen + gin) and “hydrogin” (hydrogen + gin) are jocular terms. A now-classic joke was printed in Manners: A Novel by Miss Francis Moore:
 
“Every body learns chemistry in town. I always attend the Royal Institution;—Sir Humphrey Davy is so dear! so animated! so delightful! I once asked him, ‘My dear Sir Davy,’ says I, ‘what’s the distinction between oxygen and hydrogen?’ ‘Why,’ says he, ‘one is pure gin, and the other is gin and water.’”
 
This version was printed in the Caledonian Mercury (Edinburgh, Scotland) on August 30, 1823:
       
“At Dublin, on Saturday evening, while Mr Charles, the ventriloquist, was describing the nature of gas, a blue stocking lady very clamorously inquired of a gentlemen near her what he meant of oxy-gin and hydro-gin, or what was the difference. Very little, Madam, said he—by oxy-gin, we mean pure gin, and by hydro-gin, gin and water.”
       
The joke is usually attributed to a student examination answer. This was printed in the Hartford (CT) Courant on January 26, 1926:
     
“One youth wrote that ‘Water is composed of two gins—oxygin and hydrogin. Oxygin is pure gin, hydrogin is gin and water.’”
     
         
Wikipedia: Water
Water (chemical formula H2O) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth’s hydrosphere and the fluids of all known living organisms (in which it acts as a solvent). It is vital for all known forms of life, even though it provides no calories or organic nutrients. Its chemical formula H2O, indicates that each of its molecules contains one oxygen and two hydrogen atoms, connected by covalent bonds.
 
Google Books
Manners:
A Novel (Volume I)
By Miss Francis Moore
London, UK: Baldwin, Cradock, and Joy
1817
Pg. 87:
Every body learns chemistry in town. I always attend the Royal Institution;—Sir Humphrey Davy is so dear! so animated! so delightful! I once asked him, ‘My dear Sir Davy,’ says I, ‘what’s the distinction between oxygen and hydrogen?’ ‘Why,’ says he, ‘one is pure gin, and the other is gin and water.’
         
Newspapers.com
30 August 1823, Caledonian Mercury (Edinburgh, Scotland), pg. 4, col. 4:
At Dublin, on Saturday evening, while Mr Charles, the ventriloquist, was describing the nature of gas, a blue stocking lady very clamorously inquired of a gentlemen near her what he meant of oxy-gin and hydro-gin, or what was the difference. Very little, Madam, said he—by oxy-gin, we mean pure gin, and by hydro-gin, gin and water.
 
Newspapers.com
8 May 1824, New England Farmer (Boston, MA), pg. 328, col. 2:
A lecturer on Chemistry was describing the nature of some of the gases, when a lady present, asked the gentleman who attended her what was meant by the terms oxygen and hydrogen, and what was the difference. Very little, Madam, said the gallant. By oxy-gin, we mean pure gin; and by hydro-gin, gin and water.
     
Google Books
Something for Everybody
By John Timbs
London, UK: Lockwood and Co.
1861
Pg. 199:
A scientific wag has defined oxygen to be pure gin, and hydrogen gin-and-water
 
Newspapers.com
26 January 1926, Hartford (CT) Courant, “Erring Definitions,” pg. 12, col. 3:
One youth wrote that “Water is composed of two gins—oxygin and hydrogin. Oxygin is pure gin, hydrogin is gin and water.”
 
Newspapers.com
12 February 1926, Brooklyn (NY) Daily Eagle, “Classroom Chuckles,” pg. 3, col. 2:
A pupil in a Queens school, in the fifth grade, describes water as follows: “Water is composed of two gins—oxygen and hydrogen. Oxygen is pure gin and hydrogen is gin and water.”
 
Contributed by Grace D. Griffing, 46 39th st., Corona, L. I.
 
Newspapers.com
8 January 1929, The Home Journal (Murfreesboro, TN), “Strange Ideas Prevail Regarding Some Things,” pg. 6, col. 2:
Here are a few of the “bright” thoughts contributed in examination questions:
(...)
Water is composed of two gins, Oxygin and hydrogin. Oxygin is pure gin; hydrogin is gin and water.
 
Twitter
Brian@bphuff·Jan 13, 2009
Replying to @gazellewc
@gazellewc “Water is composed of two gins, Oxygin and Hydrogin. Oxygin is pure gin. Hydrogin is gin and water.”
     
Google Groups: jokes-n-toons
March 3rd Jokes-N-Toons
SJ JNT
Mar 3, 2009, 5:57:14 AM
(...)
Science Definitions from Kids
(...)
Water is composed of two gins, oxygin and hydrogin. Oxygin is pure gin. Hydrogin is gin and water.
 
Twitter
Ankit Desai
@dotcoy
“Water is composed of two gins, oxygin and hydrogin. Oxygin is pure gin. Hydrogin is gin and water.” - Noted… http://tumblr.com/xsr1e1rpq
7:23 AM · Mar 6, 2009·Twitter Web Client
   
Twitter
Severus Snape
@SSnape
“Chemical formula of water has two gins - oxygin and hydrogin. Oxygin is pure gin while hydrogin is gin and water.” -Daft 1st year. Really?
10:35 PM · Apr 29, 2009·Twitter Web Client
   
Twitter
Quentin Quarantino
@marcuss76
Water is composed of two gins, Oxygin and Hydrogin. Oxygin is pure gin.
Hydrogin is gin and water. I prefer gin and lime.
11:28 AM · May 8, 2009·Twitter Web Client
   
Twitter
Sudarshan Banerjee
@additiyom
“Water is composed of two gins, Oxygin and Hydrogin. Oxygin is pure gin. Hydrogin is gin and water.” LOL, over from #FB via @esgee54
3:12 AM · May 22, 2009·Twitter Web Client
   
Twitter
Tweepak
@DeePakao
Water is composed of two gins - Oxygin & Hydrogin. Oxygin is pure gin. Hydrogin is gin and water! H20 is hot water, CO2 is cold water! 😉
10:21 AM · May 26, 2009·Twitter Web Client
 
Reddit—Funny
Posted by u/Satzaroo December 27, 2013
Alleged answers on science tests
When you smell an odorless gas, it is probably carbon monoxide.
 
Water is composed of two gins, oxygin and hydrogin. Oxygin is pure gin. Hydrogin is gin and wateR
 
Twitter
Bob Kostic
@causticbob
Water is composed of two gins, Oxygin and Hydrogin.
Oxygin is pure gin.
Hydrogin is gin and water.
6:48 AM · Mar 18, 2016·Twitter Web Client
 
Twitter
Yogesh Leela Pawar
@powerofyogesh
Gems from actual test papers…...
1. Water is composed of two gins, Oxygin & Hydrogin. Oxygin is pure gin. Hydrogin is gin and water.
2. Blood flows down one leg and up the other.
3. Dew is formed on leaves when the sun shines down on them and makes them perspire.
11:37 AM · Jul 2, 2021 from Mumbai, India·Twitter for Android