Mushrooms are grown by packing sawdust into airtight bags, then steam cooking the packed bags for several hours. After these pasteurized wood chips have cooled down small pieces of mushroom tissue are introduced into the bag, which eagerly devours the neutralized wood. As the fungus digests and transforms the contents of the bag it solidifies into a mass of interlocking cells, slowly becoming denser and taking form. Like plaster or cement, mushrooms can be cast into almost any shape.