Step-by-step explanation:
As the pickle jar was placed inside the refrigerator so, due to the decrease in temperature of the refrigerator the lid faced contraction in size.
As a result, it became tightly fitted over the mouth of the jar.
But when the lid was held under the hot water from your sink's faucet for a few seconds, then it undergoes a thermal expansion.
As a result, size of lid expanded as compared to the size of mouth of the jar. Hence, the lid is able to be opened up.