What is the scientific explanation on why glass breaks when you heat it then cool it immediately?

Question

I'm trying to help my 10 years old niece for her Science Fair. What is the term used to explain this occurence? Would you say thermal conductivity? Thank you.

Answer 1

Almost any solid expands when heated and shrinks when cooled (there are very few exceptions to that rule). When you quickly cool a piece of glass, the layers that are in immediate contact with the cooling agent shrink, but the deeper layers don't quite catch up, since they are still warm. Tension results, and the glass may crack or break.

The technical term for this is "thermal tension" and, as you can see from the explanation above, it is caused not by thermal conductivity, but rather, by the lack thereof.