Halakhah for Beitzah 2:10
עֲגָלָה שֶׁל קָטָן טְמֵאָה מִדְרָס וְנִטֶּלֶת בְּשַׁבָּת, וְאֵינָהּ נִגְרֶרֶת אֶלָּא עַל גַּבֵּי כֵלִים. רַבִּי יְהוּדָה אוֹמֵר, כָּל הַכֵּלִים אֵין נִגְרָרִין חוּץ מִן הָעֲגָלָה, מִפְּנֵי שֶׁהִיא כוֹבֶשֶׁת:
A child's carriage [made for a child to play with and set aside for him to sit upon] acquires midras ("treading") uncleanliness [If the child were a zav (one with a genital discharge), the carriage becomes av hatumah (a "progenitor" of uncleanliness) ], and it may be moved on Shabbath, [for it has the status of a vessel]. But it may not be rolled [on Shabbath], except on garments, [for it makes a groove in the ground, and one who "digs" is liable by reason of "plowing."] R. Yehudah says: All appurtenances may not be rolled (on Shabbath), except a carriage, because it presses. [Two tannaim (rule here) in accordance with R. Yehudah, the first also representing the view of R. Yehudah, who says that "something not intended is forbidden." And the other tanna comes to apprise us that R. Yehudah did not forbid a child's carriage, because it does not make a groove by "digging," as it rolls, but it presses down the soil beneath it without dislodging it. The halachah has already been stated in accordance with R. Shimon, viz.: one may drag a bed, a chair, and a bench so long as he does not intend to make a groove.]