"Telling The Time" joke

An American couple, vacationing with some friends in Mexico, were shopping at the market for some souvenirs. Time passed, and they realized that neither of them was wearing a watch. They noticed a little Mexican man taking a siesta next to a mule, which happened to have the largest set of mule nuts they had ever seen.
Doing their best to avoid staring at the enormous mule nuts, they asked the little Mexican man if he could tell them what the time was.
The little Mexican man reached his hand under the huge set of mule balls and, lifting them high, said, "It's 3 o'clock."
Amazed by this, the American couple ran off to find their friends to tell them what they had just witnessed. "You aren't going to believe this," they told their friends, "but, there's a little Mexican man over there who can tell the time by lifting his mule's balls!"
Curious, although somewhat skeptical, the friends wanted to see this first-hand, so they went back and asked the little Mexican man what time it was.
Sure enough, the little Mexican man reached out, cupped his hands under the mule's balls, lifted them up as if to weigh them, and said, "It is 3:30."
The friends checked the time on their watches and were astounded to see that the little Mexican man was right.
"This is so amazing! How do you do that?" they asked.
"Do what?" asked the little Mexican man.
"Tell the time by lifting your mule's balls!" they said in unison.
"Ah... I just need to lift his balls so I can see that big clock across the street," he explained.