[chris (OP)]

Posted on behalf of Lois...

Firstly I love your podcast - https://www.thenakedscientists.com/podcasts. I listen while I do patchwork.

My standard way of opening  a stubborn metal lid on a glass jar for the first time is to run hot tap water over the lid. I think the jar opens more easily because the lid expands with the heat. My husband thinks the heat makes the air in the jar expand and this pushes the lid off.

Why does the lid come off more easily ?

a. lid expansion
b. air expansion
c. a combination of a. and b.
d.something else

Cheers Lois

[vhfpmr]

Both. Steel expands at twice the rate of glass, so the lid will get looser at it heats up, and jars are usually sealed with a partial vacuum to keep the security button down, in which case expansion of the air will help reduce the pressure difference that's holding the lid on.

[alancalverd]

(a), (b), (c) and (d) - softening of any plastic/paint/honey that may be sticking the lid to the jar and preventing it rotating.