Diffusion is a principle in Physics, Chemistry, and Biology. The rate of diffusion is affected by temperature, particle size, concentration, and material type. Students can model the rate of diffusion based on particle size by contrasting blue and yellow dyes. Two petri dishes containing agar-agar receive a drop of dye at the center. The radius of expansion is recorded over time. The variance of the distribution grows as 𝜎2=4𝐷𝑡, where 𝜎2 is the variance, D is the diffusion constant, and t is time. Graphing variance versus time gives a slope of 4D. Diffusion constants vary by particle size, allowing for a size ratio comparison between blue and yellow dyes. Relating this to cells, students predict that smaller molecules diffuse into living cells, whereas larger molecules need some assistance from protein channels as in facilitated diffusion. In Physics and Chemistry, the data can be related to thermal energy.