Where the line gets drawn between a solid and a liquid has always been a slightly slippery slope. Not as slippery as the AppStore approval system, but still the TSA aren’t really doing a fantastic job. XKCD puts it rather well in regard to what you should and should not be allowed to take on a plane.

The general rule for a solid vs. a liquid state is this:

Peanut butter in the jar is a liquid.
Peanut butter spread on bread is a solid.

That’s right. The same substance can count as a solid or as a liquid, depending on how your transport it.

Anyhow, the general tip for thanksgiving is to leave the leftovers in the fridge. Mashed potatoes count as liquid. The food channel published an article on this topic and to me the mashed potatoes really do stand out.