ציטירנדיקער פּירוש אויף ויקרא 15:4

ומבחר AND THE CHOICEST — This word is a noun of similar form to מֶרְכָּב and מִשְׁכָּב and מִקְרָא [קדש]‏.

A child’s cart is susceptible to the impurity of midras, and it may be handled on Shabbat, and it is dragged along only on matting. A child’s cart is one that a small child would push around while learning how to walk (today these make sounds and have spinning things to keep your child entertained while you fold the laundry). Since the cart is made to lean upon, it can receive a type of impurity called “midras.” “Midras” means pressure. A zav or a zavah (a man or woman with an abnormal genital discharge) who steps, sits, lies or leans upon this cart, in other words applies bodily pressure to the cart, will render it impure. This type of impurity is derived from Leviticus 15:4. The cart may be handled on Shabbat and on Yom Tov. It is not muktzeh. However, it is forbidden to drag it along the ground because it will make grooves in the ground. This is a forbidden labor on Shabbat and on Yom Tov which is derived from the forbidden labor of plowing. The only way to use it is to drag it on some type of matting.

According to Leviticus 15:4, any object on which a person who has had a discharge sits is impure. The Rabbis taught that things that are not made to sit upon or are not fit to sit upon, do not receive impurity. Our mishnah discusses two types of stools and whether or not they can receive impurity.