Reversible binding between biomolecules—for example, between a cell-sruface receptor such as the insulin receptor and its corresponding natural ligand such as insulin—is central to innumerable physiological transactions. Binding of the dye HABA to egg-white avidin is a simple, reliable, and colorful laboratory model for introducing beginning biology students to the principles underlying reversible binding. They can probe the reaction quantitatively with a spectrophotometer, and model it mathematically using only high-school algebra and a spreadsheet program such as Microsoft Excel.