This lesson introduces how to calculate an arithmetic series, specifically Fibonacci. In the first of two hour-long sessions, using a spreadsheet (e.g. Microsoft Excel or Google Drive Sheets), students are shown how to calculate a series based on two prior values (the iterative solution), and by using a user-defined function (the recursive solution). With a large enough domain, most computers will exhibit real delays in calculating the recursion for values greater than 30. In the second session, they will explore why the iterative solution is faster, and why the recursive solution significantly slows down for large values. This lesson assumes that the teacher is well versed in using spreadsheets, including copy-down formulas.