When canning, put the flat disc part of your jar lids into a heatproof bowl making sure they’re not stacked tight. After you’ve boiled the jars, pour some of the hot water into the bowl. This will soften the sealing compound under the lids, allowing it to form a tight bond with the glass jars. Boiling the lids should be avoided, since it could damage the sealant. We came across this handy tip while following recipes from Canning For A New Generation, by Liana Krissoff.