Once there was a farmer who a farmer who couldn't
distinguish between his two horses. Since he didn't
know what to do he asked his neighbor for an advice.

"Why don't you trim the tail of one of the horses,"
advised the neighbor.

"You are a very clever man" said the farmer.

For a couple of months it was all right, but then the
tail grew back to its normal size.

"You'd better trim the mane of one of the horses,"
advised again the neighbor.

For another couple of months it was ok, but then the
mane grew back.

"I think the best thing to do is to look for something
characteristic that doesn't vary with the time, like the
height of the horses," said the neighbor.

"You are a very wise man, I'll measure it and tell you
about it."

After a few hours the farmer returned to his neighbor:

"You were perfectly right. I've measured their height
and indeed the white horse is 3 inches taller than the
black one."