Colloidal particles or polymers in a sol or solution are connected to each other under certain conditions, the formation of spatial network structure, the structure gap is filled with as a dispersing medium of liquid (in the dry gel can also be gas, dry gel is also known as aerogels), such a special dispersion system called gel. No mobility. The interior often contains a large amount of liquid. For example, blood gel, agar water content can reach more than 99%. Can be divided into elastic gel and brittle gel. When the elastic gel loses the dispersing medium, the volume shrinks significantly, and when the dispersing medium is absorbed, the volume expands again, such as gelatin and so on. When the brittle gel loses or absorbs the dispersed medium, the shape and volume are unchanged, such as silica gel. The process of forming a gel by solution or Sol is called a gelling effect (gelation).