The students in 5th grade recently started a unit in physical science entitled "Variables". The unit begins with an investigation into the concepts of *capacity* and using *variables *in a controlled experiment. Students constructed standard paper-cup boats and tested them "at sea" to find out how many passengers-pennies-they can carry without sinking. After identifying variables that influence the number of passengers they can carry, they construct three more boats of differing sizes and determine the capacity of each boat by measuring the volume of water each can hold. At the completion of the investigation, the students come away with an understanding that *capacity* is the amount of fluid a container can hold and that *variables* are things you can change that might affect the outcome of an experiment.