He took out of his pocket a tin-opener which he usually carried about with him. It was the sort with a buttery handle which you had to turn. Just to show, he fitted it on to the bottom of one of the empty tins and twisted the handle. The opener crept round the edge of the tin, the blade ploughed into the metal at the bottom, and soon the shiny round disc of metal came loose.

What is the subject matter of this paragraph?

This paragraph introduces a new theme, within the chapter. The new theme is the tin-opener. 

Stig was fascinated. He looked at the at round piece of tin which had been the bottom, he looked at the empty tube which was all that was left of the rest of it. And he took the tin-opener from Barney and turned the handle, but he couldn’t make it out.

What is the subject matter of this paragraph?

The subject matter of this paragraph is to introduce a new characters thoughts, feeling and actions. Stig thoughts and actions based on the tin-opener.

‘It’s quite easy, Stig. Look!’ and Barney took another tin, fitted the opener on the bottom, and showed him how to work it. And there was another round plate and another tin tube. Then Stig had to have a go, and they started on a third.

What is the subject matter of this paragraph?

This paragraphs subject is that it introduces a new speaker. Barney begins to speak about the tin-opener.

What is the link between all 3 of the paragraphs?

The paragraphs are linked through the new theme (tin-opener) and how it interests Stig.