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Step 6 - Covering the Jars
Don't screw on the lids too tightly.
Photo © Carroll Pellegrinelli, licensed to About.com, Inc.
For the jars to seal themselves, the jars must still be hot. The lids should also be at least room temperature. Sit them on the stove, if necessary. Don't screw on the caps too tightly. Within 15 minutes of capping the jars, you'll hear popping. This is when the heat from the jars cause a vacuum making the jars airtight.