Sponges usually reproduce when little pieces break off. If such a piece has the right types of cells it can grow to become a new sponge. A few sponges can also use budding. With budding a small sponge grows on the parent; when it is done growing, it simply "falls off". When the conditions are bad, some sponges can also grow lumps of unspecialised cells. These will not develop until the conditions improve again. They can then either make a new sponge, or they can use the skeleton of the parent sponge (that died).