Looking for the stages that clarify the previous mishnah. A bed can, of course, be rendered impure and then be purified. Even if the bed is dismantled... Whereas, mats woven from various reeds and grasses - which are apparently too small. And lastly of those cases - digging into a grain pile to make a sukkah, depending on the measure of the hole you dig. Also, a new mishnah: wherein one lowers the walls of a sukkah to hover above the ground. How large the gap is will make the difference - in what amounts to a hanging sukkah. Which of course yields significant comparison to Eruvin, and specifically lavud.