“2% milk” refers to the fat content of the milk, but there’s a popular joke:
 
“If it’s 2% milk, what’s the other 98%?”
   
“I always wondered if it was 2% milk, what was the other 98%?” was cited in print in 1996. “I’m drinking 2% milk, wondering what the other 98% is…” was cited on Twitter in 2010. The joke was told on the American television sitcom Parks and Recreation in 2014.
   
“My milk is about to die. It’s at 2%” is a related saying.
 
   
Google Groups: sci.med.immunology
Did Vaccines Eliminate Polio?
Keith E Gatling
4/30/96
Fat content of 2% milk (funny)
I remember when I first saw 2% milk in the dining halls at SU, not quite knowing what it was.  I saw skim, chocolate, and 2%, and the 2% seemed normal to me, but I always wondered if it was 2% milk, what was the other 98%?
 
Years later, after I found out it referred to the fat content, Cheryl and I had a running joke about it…“2% milk.  What’s the other 98%?  SOAP!”*
 
*For those of you who have absolutely no idea why that’s funny (and it may indeed be a “you had to be there” sort of thing), it’s a reference to Procter and Gamble’s claim that Ivory Soap is 99.44% pure.
   
Google Groups: alt.fan.cecil-adams
chitlins, a return to sanity
Gary S. Callison
12/7/99
(...)
And while we’re on the subject, I’ve always been curious: 2% milk - what’s the other 98%?
 
Twitter
FunnyOneLiners
‏@Funnyoneliners
I’m drinking 2% milk, wondering what the other 98% is…
RT @mytweecwetlife
8:18 AM - 30 Aug 2010
 
Twitter
Steven Winterburn
‏@5tevenw
We have 2% milk in the fridge, no idea what the hell the other 98% is.
3:40 PM - 15 Oct 2010
 
Twitter
Greg Gerla
‏@greg_gerla
I like to drink 2% milk, but I can’t help worrying about what the other 98% is.
#iamwhatidrink
12:14 AM - 17 Sep 2011
 
reddit
If some milk is only 2%, what is the other 98%? (self.shittyaskscience)
June 23, 2013 by zack4565
 
Twitter
Jay Spitfire
‏@hawkgirl_kane
“When they say ‘2% milk’,I don’t know what the other 98% is.” #AndyListofSecrets #Parks&Recreation
8:37 AM - 14 Mar 2014
 
reddit
If only two percent of 2% milk is actually milk, what’s in the other 98% of it? (self.shittyaskscience)
March 23, 2014 by djm1997
 
reddit
If my carton is only 2% milk, whats the other 98%? (self.shittyaskscience)
February 24, 2015 by Endermod
   
Uproxx
The Andy Dwyer Lines We’ll Always Think Of When We See Chris Pratt
BY STEPHEN TOMPKINS • 02.24.15
#CHRIS PRATT #PARKS AND RECREATION #TV QUOTES
(...)
“When they say two percent milk, I don’t know what the other 98 percent is.”