Horse Farmer

Once there was a blonde farmer who couldn't distinguish between her 2 horses.  
Since she didn't know what to do she asked her 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 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 blonde farmer returned to her neighbor.  "You were 
perfectly right", she said, "I've measured their height and indeed the white 
horse is 3 inches taller than the black one."