A horse farmer walks into a bar looking depressed

The bartender says hey what's the matter?
The horse farmer says
"I have these two horses and I just can't for the life of me tell them apart"
The bartender tells the horse farmer to weigh his horses, so he goes home and does so. He returns the next day with a long face once more, and says to the bartender
"It's no good each horse has the exact same weight"
The bartender mules it over, and says
"Here's a thought, go home, take your horses, and measure their exact height. Then you'll be able to tell your horses apart by their height."
"What a great idea"
The horse farmer says. He goes home, measures his horses and comes back to the bar the next day with a face full of tears, weeping and the bar man can tell the horse farmer's horses are the same height. The bartender thinks and thinks of a way to tell these horses apart, when,
"Ah ha!" proclaimed the bartender excitedly
"Here's what you do, go home to your horses sir, and measure the lengths of your horses tails, then finally you will be able to distinguish between your horses."
"Thank you thank you! That's brilliant!" Exclaimed the newly relieved horse farmer, and on his way home he went to measure the lengths of his horses tail. This horse farmer returns to the bar once more with a big ol' grin on his face, proudly announcing to the bartender,
"Great news! It worked! My white horse has a tail 1/3 of an inch shorter than my black horse!"