Question: Why does bread sink when you open the oven to check on it ?

Elaine Kennedy answered on 6 Nov 2017:

When you bake bread carbon dioxide – CO2 – is produced by the yeast as it ferments the sugars in the dough. When we knead the dough we make it stiff and firm and the protein in the flour help to trap the CO2 in small bubbles. The small bubbles slowly fill with CO2 as the bread bakes and this makes the bread rise. If you don’t knead the tough enough it can sink when you open the oven.
If you knead the dough until it becomes very stiff and let the bread bake for a while before you check it (try to avoid opening the oven too often) then it should not sink.