There are countless small holes in tofu. These holes vary in size, some are interconnected, and some are closed into small "containers". These holes are filled with water. We know that water has a strange characteristic: at 4℃, its density is the largest and its volume is the smallest; Ice is formed at 0℃, and its volume is not reduced but expanded, which is about 10% larger than that of water at room temperature. When the temperature of tofu drops below 0℃, the water inside becomes ice, the original pores are enlarged by ice, and the whole tofu is squeezed into a net. When ice melts into water and runs away from tofu, it leaves countless holes, making tofu look like foam.