Keilim, Chapter 24, Mishnah 8. This chapter is built out of a long series of short rulings, each one opening with the words "shalosh... hen," there are three of a given utensil. In every case the same familiar name covers three different objects, and the halachah of each one depends on what it was actually built to do. One version receives tum'at midras, a second receives only tum'at met, and a third is tahor altogether. Our mishnah applies that pattern to the bed.
Our mishnah opens with the words "shalosh mitot hen." The Hebrew word mitah is used for three separate items, and the halachah assigns each of them its own status in the laws of tum'ah.
A bed intended for resting
"Ha'asuyah lishchivah teme'ah midras": when a bed was constructed so that a person could lie down and sleep upon it, it is subject to tum'at midras. This category of tum'ah takes hold of items designed to support the weight of a human body, whether by lying, sitting or reclining. Nothing illustrates that description better than a regular bed, whose sole reason for existing is to carry someone at rest. It therefore falls into the strictest classification of tum'ah.
The stand used by glassworkers
"Shel zaggagin teme'ah tum'at met": the mitah belonging to glassworkers is susceptible to the tum'ah of a corpse. Those who made and sold glass would lay out their merchandise on a wooden platform shaped much like a bed. A border ran along its edges, and a wooden article with a raised border qualifies as a container, an object capable of holding something within it. Since susceptibility in wooden vessels depends on the ability to contain, such a platform becomes tamei through a corpse present in the same room, or through contact with any other source of tum'ah.
Midras tum'ah, however, does not apply to it, for no one leans his body upon it. It is a fragile fixture covered with glass items, hardly a place where a person would take a seat. Given that it was not fashioned for resting or sitting, nor is it treated that way, midras never enters the picture, and its susceptibility stops at tum'at met.
The loom of the net weavers
"Veshel saragin tehorah mikelum": the mitah of the net weavers is free of tum'ah completely. This was the working frame upon which nets were manufactured. Cords were pulled tight across it and secured at the edges, after which additional strands were threaded among them until a finished net emerged. Once the weaving was complete, the net was removed and the frame put away, since its only job had been to keep the cords stretched while the weaver worked.
A frame of this sort is nothing more than flat wood, with no hollow space and no border, and flat wooden articles, pshutei klei etz, are never susceptible to tum'ah. Lacking any capacity to contain, and serving neither as a seat nor as a place to lie, it stays tahor from every kind of tum'ah.
Three objects, one name, three different halachot. The bed built for a person's rest carries the severity of midras; the glassmaker's rimmed stand is a receptacle and so receives tum'at met alone; and the net maker's bare frame, holding nothing and bearing no one, stays tahor from everything.