Mishnà su Ohalot 15:17

The following are not susceptible to impurity among earthen vessels: A tray without a rim, A broken incense-pan, A pierced pan for roasting corn, Gutters even if they are bent and even if they have some form of receptacle, A cooking vessel that was turned into a bread-basket cover, A bucket that was turned into a cover for grapes, A barrel used for swimmers, A small jar fixed to the sides of a ladle, A bed, a stool, a bench, a table, a ship, and an earthen lamp, behold these are no susceptible to impurity. The following is a general rule: any among earthen vessels that has no inner part is not susceptible to impurity on its outer sides.

A table one of whose legs was removed is clean. If a second leg was removed it is still clean. But if a third was removed it becomes unclean where the owner has the intention of using it. Rabbi Yose says: no intention is necessary. The same law applies also to the side-board.