i am looking for a particular aesop's fable about a blind man and a
boy sharing grapes.  they agree to take turns having one apiece.  at
some point the blind man starts taking 2 on his turn.  the boy notices
this and after a few turns starts taking 3 at each of his turns. 
suddenly the blind man says, "You're cheating!"  and the boy, shocked,
says: "How did you know?"  And the blind man replies, "Because you
didn't complain when i started taking two."

something like that

not entirely sure it's grapes

not entirely sure it's aesop, but it seems likely

Hi Barma,

I found the story you are looking for. It is not actually just a
single story but a part of a much longer story -- a whole chapter in
the life of a person named Lazarillo. You can view it online from one
of the pages of the University Libraries of the University of
Maryland. Below is the excerpt of that particular story:

" And it happened that as we were coming to a place called Almorox
when they were gathering the grapes, a grape picker gave him a bunch
as alms. And since the baskets are usually handled pretty roughly and
the grapes were very ripe at the time, the bunch started to fall apart
in his hand. If we had thrown it in the sack, it and everything it
touched would have spoiled. He decided that we'd have a picnic so that
it wouldn't go to waste -- and he did it to please me, too, since he'd
kicked and beat me quite a bit that day. So we sat down on a low wall,
and he said: "Now I want to be generous with you: we'll share this
bunch of grapes, and you can eat as many as I do.  We'll divide it
like this: you take one, then I'll take one. But you have to promise
me that you won't take more than one at a time. I'll do the same until
we finish, and that way there won't be any cheating."

The agreement was made, and we began. But on his second turn, the
traitor changed his mind and began to take two at a time, evidently
thinking that I was doing the same. But when I saw that he had broken
our agreement, I wasn't satisfied with going at his rate of speed.
Instead, I went even further: I took two at a time, or three at a time
-- in fact, I ate them as fast as I could. And when there weren't any
grapes left, he just sat there for a while with the stem in his hand,
and then he shook his head and said, "Lazaro, you tricked me. I'll
swear to God that you ate these grapes three at a time."

"No, I didn't," I said. "But why do you think so?"

That wise old blind man answered, "Do you know how I see that you ate
them three at a time? Because I was eating them two at a time, and you
didn't say a word."

I laughed to myself, and even though I was only a boy, I was very much
aware of the sharpness of that blind man. "

(Excerpted from "Chapter 1: Lazaro Tells about His Life and His
Parents" http://www.lib.umd.edu/ETC/ReadingRoom/Nonfiction/Lazarillo/LifeLazarillo/chapter01.txt)

Source:

University Libraries, University of Maryland
"The Life of Lazarillo of Tormes his fortunes and misfortunes as told
by himself - Translated by Robert Rudder"
http://www.lib.umd.edu/ETC/ReadingRoom/Nonfiction/Lazarillo/


Search strategy:

"blind man" boy share grapes two cheating
://www.google.com/search?num=100&hl=en&lr=&ie=ISO-8859-1&newwindow=1&safe=off&q=%22blind+man%22+boy+share+grapes+two+cheating

I hope this helps you. Should you have any comments/questions, please
feel free to post your clarification before rating this and I'll
attend to you as soon as possible. Thanks for asking.

Best regards,
Feilong

great.  i don't remember when or where i heard it, but that has to be
it.  thanks for prompt answer