Students measure energy outputs and inputs to determine the efficiency of conversions and simple systems. One associated activity includes LEGO® motors and accomplishing work. The other investigates energy for heating water. Students learn about by-products of energy conversions and how to improve upon efficiency. Conduct either or both activities. The calculations in the water heating experiment are more complicated than in the LEGO motor activity, making the heating activity more suitable for older students, and only the LEGO motor activity suitable for younger students.