The document discusses the three types of variables in scientific investigations: independent variables, which are manipulated by the scientist; dependent variables, which are observed and measured; and controlled variables, which are kept the same. It provides examples of investigations where students' ages, sizes of nails, water temperature, and water depth are independent variables, and time to complete a puzzle, number of paper clips picked up, time for an egg to boil, and water temperature are dependent variables. The puzzle, materials in the electromagnetic system, type of egg, and thermometer are examples of controlled variables.