Beitzah, Chapter Two, Mishnah 10. This Mishnah, too, continues with matters of tumah and taharah.
"Agalah shel katan":
This refers to a kind of small walker, a walking toy for a child who is learning to walk. The child would hold on to it, and it would move and roll along in front of him on its wheels.
Three laws are stated regarding it:
"Temei'ah midras" - the wagon is susceptible to midras tumah.
"V'niteles b'Shabbos" - it may be moved on Shabbos, since it is considered a utensil and is not muktzeh: it has a function and is not set aside from use.
"V'einah nigreres ela al gabei kelim" - on Shabbos and Yom Tov it may be dragged only over garments, a rug or the like, and not over bare ground.
What is midras tumah?
The law of a zav is that anything he sits on, treads on, walks on or lies on becomes tamei with midras tumah, and becomes an av hatumah capable of conveying tumah to other utensils, to people and to food. Therefore, if the child was a zav and leaned on the wagon or sat on it while riding, the wagon takes on this status. One of the criteria for midras tumah is that the utensil be designated for sitting or leaning, and a child's wagon is indeed designated for that.
"V'einah nigreres ela al gabei kelim" - an unintended act:
Even though the wagon is dragged and is not deliberately digging, the Mishnah was concerned that it might create a groove or furrow in the ground. Our Mishnah follows the view of Rabbi Yehudah, who holds that a "davar she'eino miskavein" (an unintended act) is forbidden: even though the person has no intention of making a furrow in the ground, since there is a reasonable possibility that it will happen in the course of the action, it is forbidden. Therefore this utensil may not be dragged over bare ground.
The view of Rabbi Yehudah:
Rabbi Yehudah says: "Kol hakeilim ein nigrarin chutz min ha'agalah, mipnei shehi koveshes" - no utensils may be dragged, except for this wagon. Even when it does make a groove, there is no actual digging here, only a pressing of the earth downward, and pressing earth down is not considered a form of building. Since there is no act of digging here but only pressing downward, it is permitted.
The following point is worth noting: both the view of the Tanna Kamma, that an unintended act is forbidden, and the permission to drag the wagon, both represent the position of Rabbi Yehudah. At times the Tannaim themselves disagree as to what Rabbi Yehudah held in a particular case: according to one view, Rabbi Yehudah's position is that an unintended act is forbidden, and therefore this utensil may not be dragged; and against it another view is presented in the name of Rabbi Yehudah, that indeed one may generally not drag, but this particular utensil is permitted, because it does not perform an act of digging but only an act of pressing downward.
In summary: in this Mishnah we learned three laws regarding a child's wagon: it is susceptible to midras tumah because it is designated for sitting and leaning; it may be moved on Shabbos, since it is a utensil and is not muktzeh; and it may be dragged only over utensils, out of concern for making a groove in the ground - following the view of Rabbi Yehudah that an unintended act is forbidden. Rabbi Yehudah permits dragging specifically this wagon, because it presses the earth down and does not dig, and we examined the disagreement among the Tannaim in understanding his position.