We describe a simple field study that we have found useful in introducing students to experimental design. Students manipulate the nutritive gain available from flowers to test the hypothesis that the foraging behavior of nectarivorous insects maximizes energy gain rate. They add sucrose solution to some flowers and water to others; additional flowers are left unmanipulated. Visit durations of foraging butterflies are then measured to test the prediction that individuals will forage longer at patches that offer higher energy gains. The project encourages students to consider how a study's design influences the results obtained, and helps to develop scientific reasoning skills.