Baseball umpires frequently dust off home plate. A baseball riddle is:
 
Q: Why are many umpires fat?
A: Because they always clean their plate.
 
The riddle has been cited in print since at least 2006.
   
       
Google Books
Jokelopedia:
The Biggest, Best, Silliest, Dumbest, Joke Book Ever
Compiled by Ilana Weitzman, Eva Blank, Alison Benjamin and Rosanne Green
New York, NY: Workman Publishing
2006
Pg. 77:
Are baseball umpires good eaters?
Yes — they always clean their plates.
 
Google Books
March 2009, Boys’ Life, “Think & Grin,” pg. 56, col. 2:
Michael: Why are some umpires fat?
Andrew: Tell me.
Michael: They always clean their plate!
Noah Brittenham, Lincoln, Neb.
   
Google Groups: trevs-daily-family-post
TREV-: Jest For Kids 08-03 10
theoldfella
8/3/10
(...)
Are baseball umpires good eaters?
Yes! They always clean their plates.
 
Google Books
Jokes That Flip Your Lid
By Jerry J. Mercatelli
Xlibris Corporation (Xlibris.com)
2011
Pg. 7:
How come umpires are never hungry?
Because they always clean their plate.
 
Twitter
Likovic
‏@audreey_10
Adonis : Why are Umpires fat? Kids : Why?! Adonis : Because the like to clean their plate. Nikko : Dude thats lame! xD
7:28 PM - 22 Sep 2011
     
The Baseball Bloggess
Posted on January 26, 2014
There’s No Plate Like Home.
(...)
(You may insert your favorite umpire joke here. Or, try this one … Why are umpires so fat? They always clean their plates!)
(...)
* Umpires have their own very specific rules for the care of home plate. They will dust it with a brush before each half inning and whenever needed. The umpire will step to the front of the plate, turning his back to the pitcher’s mound before dusting, so as not to moon the fans when he bends over. Players don’t dust off the plate. Ever.
   
Google Books
111 LOL Jokes About School & Wimpy Teachers!:
Jokes, Riddles and Crazy Excuses for Avoiding Homework!
By Marcus Stuart, Sir Laughs A. Lot
LOL Kids Books
2015
Pg. ?:
Why are so many umpires fat?
Because they always clean their plate!
 
Google Books
Playin’ Catch With Dad
By Christopher R. Ford
BookBaby
2016
Pg. ?:
“Why are some umpires fat?”
“I don’t know.” Me and my brother say.
“Because they always clean their plate.”
 
Twitter
Fire Eye Umpire
‏@FireEyeUmpire
Coach: Why are some umpires fat?
Player: I don’t know
Coach: Because they always clean their plate!
#baseball
5:14 PM - 30 Aug 2016