"A Pig who Wanted to Get Weighed" joke

(This one was on the radio this morning. I didn't hear who
the originator was, so I can't give credit where it's due.)
So this guy wants to have a luau.
He needs a pig for a luau, so he goes to a pig farm.
He asks the farmer for a twenty-pound pig.
The farmer goes into the pen, searches around awhile.
He picks up a pig, puts the tail in his mouth, and begins
swinging the pig around for a few seconds.
He puts the pig down, and says, "Nope, not quite twenty pounds."
He picks up another, puts the tail in his mouth, swings
the pig around awhile, and declares, "This one's twenty pounds!"
He brings the pig out, and the man says in a shocked tone,
"You can't weigh a pig like that!"
"Sure I can," said the farmer, "Watch this."
He called his son over and asked him to weigh the pig.
The boy came over, picked up the pig, put its tail in his mouth,
and swung it around awhile. He put the pig down and said, "This
one weighs twenty pounds."
The man still looked perplexed, so the farmer told the boy to
get his mother so that she can weigh the pig.
After five minutes, the boy returned alone. "She can't come out
just yet," the boy said. "She's weighing the mailman."