"Of the animals that move about on the ground, these are unclean for you: the weasel, the rat, any kind of great lizard, the gecko, the monitor lizard, the wall lizard, the skink and the chameleon.... If one of them falls into a clay pot, everything in it will be unclean, and you must break the pot." Leviticus 11:29-30, 33

Many of the spiritual ceremonies and rituals that God gave the Israelites had to do with sanitation and health, and this is one of them.

Because of the nature of these animals, they carried many germs, and so God declared them unclean. Clothing that was touched by them was to be washed, and because of their porous nature pots were to be destroyed. The porous pots easily absorbed any uncleanness and thus washing or scouring wasn't sufficient to clean or purify them.

An interesting note- if the clay pots were used for cooking "holy" meat, they must afterwards be broken, but a bronze non-porous pot could be cleaned: "The clay pot the meat is cooked in must be broken; but if it is cooked in a broze pot, the pot is to be scoured and rinsed with water" (Lev.6:28)