Sukkah, Chapter 1, Mishnah 8. The Mishnah discusses someone who made a roof for his sukkah - and in this case "roof" means the sechach - using spits or the long side-boards of a bed. Neither of these is valid sechach on its own.
The two types of invalid sechach in the Mishnah:
"Shefudin" - metal rods. Since they are made of metal, they are not something that grows from the ground and do not come from the ground, and they are therefore invalid as sechach.
"Arukos hamitah" - the long poles that hold the frame of the bed together. These are invalid as sechach because they are a utensil that is susceptible to tumah.
The solution - space between them:
The Mishnah says: "Im yesh revach beineihen k'motan - kesheirah" - if there is a space between them equal to their own width, the sukkah is valid. That is, when a person leaves a space between the spits equal to their width and fills it with valid sechach, the sukkah is valid. Some explain that even though the Mishnah says "k'motan," which implies that half invalid sechach and half valid sechach is enough, the intent is that the space must be made slightly larger than the invalid sechach, since the valid sechach must be somewhat greater than the invalid.
"Hachotet b'gadish" - ta'aseh v'lo min he'asuy:
The Mishnah continues with a further law that relates to a principle we have already dealt with: "ta'aseh v'lo min he'asuy" - you shall make it, and not from what is already made. A person must make a sukkah that is valid at the time he makes it, and not have the sechach already lying there and only afterward become valid.
This is the ruling of "hachotet b'gadish la'asos bo sukkah - einah sukkah" - one who hollows out a space in a haystack in order to make a sukkah in it has not made a valid sukkah. Even though the hay itself is valid as sechach and can even serve as the walls of the sukkah, his sukkah is not valid, because the sechach was lying there before there were any walls at all. What is required, then, is that there first be walls of a minimum height, and only afterward should the sechach be placed upon them, so that it will be considered valid sechach.
How much of the walls must be in place when the sechach is laid down?
It should be clarified that there is no need for walls ten tefachim high at the moment the sechach is placed. Even if there was there a hollow space only one tefach high, but with the width and length of a valid sukkah - seven tefachim - there is already an "ohel" here that the sechach covers, and it is considered sechach. Afterward it is enough to raise the walls and make them valid. As long as there is a tefach over which the sechach spreads and serves as a covering for it, there is no problem of "ta'aseh v'lo min he'asuy."
In summary: in this Mishnah we learned two laws - spits and the long side-boards of a bed are invalid as sechach (because they do not grow from the ground, and because they are a utensil susceptible to tumah), and the validity of the sukkah depends on there being a space between them that is filled with valid sechach; and one who hollows out a haystack does not have a valid sukkah, because of the rule of "ta'aseh v'lo min he'asuy," unless there was already a hollow of one tefach there in the dimensions of a sukkah, over which the sechach spreads.