This Instructional Activity is a part of the Lesson 4, Unit 5, Grade 8. Consider two functions we discussed in previous lessons: the function that assigns to each count of candy bags the cost of purchasing that many bags (Table A) and the function that assigns to each time value between and seconds the distance a falling stone has dropped up to that time (Table B). How do the two functions differ in the types of inputs they each admit? Solicit answers from students, and then continue with the discussion below.