What would make the flats on canning jars bend and curl during a hot water bath?

There are a few reasons why the flats on canning jars might bend and curl during a hot water bath.

- Overheating the jars. Hot water baths should not exceed 180 degrees Fahrenheit. Higher temperatures can cause the glass to soften and warp.

- Too little headspace. When there is not enough headspace at the top of the jars, the pressure inside the jars can build up and cause the flats to bend and curl.

- Improper cooling. Hot water baths should be allowed to cool down slowly and naturally. Cooling the jars too quickly can cause them to warp.

- Poor-quality jars. Some brands of canning jars are more prone to warping than others. It is important to use high-quality jars that are specifically designed for canning.