In this activity related to the human circulatory system (on page 10 of the PDF), learners observe the dispersion of a drop of food coloring in water, draw conclusions about the movement of dissolved substances, and develop explanations about the importance of organisms' internal transport systems. This activity allows learners to investigate the process of diffusion and to consider why many organisms have internal transport systems. This lesson guide includes background information, setup and management tips, and a handout.