We started bottling our syrup in old fashined style cans this year. In the past we have just put it in mason jars with dome lids, and if for some reason they did not seal properly the top would be "popped". How can I tell if the cans have sealed? It seems if I put hot syrup into the can and cap it that as it cools it will seal. Someone told me I need to tip them upside down while they cool???