It is not unusual for the lid of a jar be difficult to unscrew, especially for jars with glass bodies and metal lids which have been kept in a refrigerator. A common trick is to run hot water over the lid of the jar for a short time, making the jar much easier to open.
1. Using what you've learned in the chapter, why does this method make the jar easier to open?
2. Why is this method especially effective for glass jars with metal lids? Metal generally has a higher thermal conductivity and thermal expansion than glass - discuss how these two effects make the jar easier to open.