Teachers and mothers like to meet each other. What does the third son roll earthworm mean?

This is a dirty joke told by Yu-Ching Fei before his debut.

Yu-Ching Fei: Today is a joke about an ancient private school.

One day, Mr. Wang said to the students: There is no class today. Mr. Wang will take you hiking. After coming back, everyone will make a limerick based on what they saw and heard on the road.

On the way, two dogs were mating. A naughty student asked, Sir, what are they doing? Other students burst into laughter.

All teachers don't like naughty students very much, so the teacher plays dumb: this is called a happy meeting.

Students want Mr. Wang to play dumb with me, and I will punish him.

Walking to a lotus pond, everyone saw a turtle sunbathing on a stone, saw someone coming, plopped into the water and swam away. The naughty student asked, Sir, what is this called?

The gentleman replied: This is a rolling earthworm.

Everyone went down again and saw a family holding a funeral. The host family was heartbroken.

The naughty student asked again, Sir, what is this? Why are they crying so sadly?

Mr. Wang said perfunctorily, this is called Jingle Club.

After a while, they saw a family's house burned down and everyone was busy putting out the fire.

This student again: Teacher, what is this?

Mr. Wang said grumpily that it was a full house.

After hiking, Mr. Wang asked, "Did you have a good time today?"

Student: Happy!

Sir: Since we had a good time, let's write a limerick based on today's activities when we get back. Give it to me tomorrow.

The next day, naughty classmates handed in limerick.

Mr. A look, almost didn't grieve:

Teachers and mothers like to meet.

Three sons roll earthworms.

The annual jingle party.

Thirty nights full of houses.