If you want to open a jar with a metal top easily, you could just put the top under hot water.

how does this work ?

Based on the principle of the coefficient of thermal expansion. Metals have a larger expansion than glass, so heating the top causes it to expand more than the glass underneath, thereby reducing the friction between the two pieces, and making it easier to open the jar.

Heat expands the metal top quicker than the glass jar. As such, it should loosen the top and make it easier to unscrew.

For more reading try: http://www.physlink.com/education/askexp…