Other examples of the power and insidiousness of exponential growth are the lilly example (Meadows et al. 1972, p. 29) and the "secret of the Persian chessboard" (Meadows et al. 1972, p. 29 and Sagan 1989). In the former example, the following question is posed: If a lilly floating on a pond doubles in size every day and, after 30 days will cover the entire pond, on what day will it cover half the pond? Although most people answer "the 15th day," the correct answer is the 29th day. --- MORE --->