I cannot imagine that you are going to be able to get 20 pages out of talking about a colloid but I will help you understand what a colloid is. First let's back up a bit. A mixture is a combination of two or more chemicals. There are two basic kinds of mixtures. A homogeneous mixture is a mixture with a single visible phase (like salt dissolved in water). A heterogeneous mixture has more than one phase (sand sitting in a glass of water). A colloid is a special type of heterogeneous mixture. It consists of two phases, a dispersed phase mixed into a medium phase. But a colloid is a mixture only on the microscopic level. To the naked eye, you cannot see the two different mediums. Only under a microscope would you see the two different components. A perfect example is milk. It looks like a white liquid. But it really is a microscopic dispersion of fat molecules in water. So for a photo, just use a glass of milk.