The Buddha compared people to four kinds of clay vessels. One type of vessel has holes in the bottom. We can pour in as much water as we like and it runs right out. When this type of person hears the Dharma, it goes in one ear and out the other. The second type of vessel has cracks. Though we pour in the Dharma, it seeps out slowly until the vessel is empty again. The third vessel is full to the brim with stale water--views and opinions. One can't pour anything new in, everything is already known. The only useful vessel is the fourth, without holes or cracks and totally empty.