These wonderful pressure sensitive seals help prevent leaking of fluids from plastic jars and provide the manufacturer with the peace of mind that their product's quality cannot be compromised prior to its sale. They also make product returns safer and more hygienic. Simply place the liner in an unlined jar cap with the "Sealed for your Protection" side facing the lid, then tighten the lid onto a jar with strong force and the liner will be sealed to the jar. Ensure there is no moisture first around the mouth and threading of the jar.