Gongsun Long was a famous scholar who lived in the State of Zhao during the Warring States Period (475-221 BC). He kept a circle of skilled people around him and often said, "A wise man should welcome anyone with a specialty."
One day, a man dressed in tattered and dirty clothes came to see him. "I have a special skill," the man said.
"What is it?" asked Gongsun.
"I have a loud voice and I'm good at shouting."
Gongsun turned to his followers and asked, "Who is good at shouting?" None of them answered. So, the scholar took the man in.
Some days later, Gongsun and his followers went on a trip. They came to a wide river and found the ferryboat was on the other side. No one knew what to do. Suddenly, Gongsun thought of the shouting expert. "Can you have a try?" he asked.
The man realized it was his chance to show his skill. He shouted to the ferryman as loud as he could, "Hey, ferryman, come here! We want to cross the river!" As his voice faded, the ferryman came to fetch them. Gongsun was very satisfied with his new follower.
Later, people used this story to describe anyone who has a special professional skill.