When a virus encounters an antibody that reacts with antigenic proteins in the viral particle, the virus and antibody clump together. When carried out under appropriate conditions, this clumping reaction will produce a visible product. Using real viruses and real antibodies to demonstrate this phenomenon is prohibitively expensive for most K-12 teachers. Less costly inorganic salts can be used to simulate the reaction. This demonstration is appropriate for life science courses that introduce the concepts of viruses, antigens and antibodies; it is also appropriate as a general demonstration of diffusion.