This is a joke meant to get some laughs:
There is John and Shirley. John and Shirley both own racehorses, and they can't tell them apart. So, John suggests to Shirley, how about if I cut my horses mane, and then we will know that the horse with the shorted mane is my horse. So Shirley says yes this is a good idea, and it worked for awhile, until John's horse's mane grew back, so the Shirley says, why don't I cut my horse's tail short and then we will know that my horse has the shorter tail? So John agrees that this is a good idea, and it worked for awhile, until Shirley's horse's tail grew back. So now they asked a blonde and the blonde came up with a solution: measure the horses and see who's is the tallest. This worked! Now they knew that Shirley's black horse was bigger then John's grey horse.
If at first you don't succeed, destroy all evidence that you tried.