In discussing heat, I did the ball/ring demo and many students were
surprised to see that the hole EXPANDS when the ring is heated. It's like
expanding a photograph of a bagel -- both the inner and outer diameters
expand. So a student today asked why actual bagels behave differently.
When the yeast is settling, the bagel hole gets SMALLER. As a class we
reasoned that the gas in the yeast releases pressure which expands the
bagel. But why is this behavior different from heating a metal ring? This
question has never come up before, so I thought I'd ask the experts
(since I don't eat bagels much!).