What precaution must you observe when hot filling the jelly into the glass jar?

When hot filling jelly into glass jars, it is crucial to ensure that the jars are pre-warmed to prevent thermal shock, which can cause them to crack or shatter. Additionally, leave appropriate headspace in the jars to allow for expansion as the jelly cools. Finally, ensure the lids are sterilized and applied tightly to create a proper seal.