Chullin 25 - What is Fit for One is Unfit for Another
What is pure in the case of earthenware utensils is impure in the case of all other utensils, and vice versa. If a dead lizard is inside a clay pot, even not touching it, the pot is impure, but if it touches the outside, the pot remains pure. The opposite is true of other utensils.
How do we know this highly unusual law of earthenware utensils? The Torah said, " an earthenware utensil into whose interior one of the eight dead reptiles will fall … will become impure " - even if the reptile does not touch it. It also said," any open utensil under the same roof with a dead body, if it does not have a cover, will become impure ", to teach that if it does have a cover, it remains pure and protects from impurity - even if it is touched from the outside.
What is pure in the case of wooden utensils is impure in the case of metal utensils, and vice versa. Wooden utensils can be used before they are fully finished, and are thus susceptible to impurity. However, if they lack a hollow of any kind, they remain pure. The reverse is true of metal utensils.
Art: Edgar Degas - Still Life With Lizard