Final answer:
Heating the jar lid under hot water causes it to expand due to the heat conduction process, making it easier to open as the metal expands more than glass.
Step-by-step explanation:
Placing the lid of a jar under hot water helps to loosen it through the process of expansion.
When the metal lid is heated, it expands more than the glass jar because metals typically have higher coefficients of expansion than glass.
This difference in expansion rates between the lid and the jar means that the lid will expand slightly larger than it was at a lower temperature, making it easier to twist off.
The correct phenomenon behind this process is conduction, where heat is transferred from the hot water to the lid by direct contact.
Once the metal lid heats up and expands, you may find that it loosens more easily due to the increased size relative to the glass jar, which does not expand as much.