Why Subtraction Is the Hardest Math in Product Design: Simple doesn't just sell, it sticks. Simple made hits of the Nest thermostat, Fitbit, and TiVo. Simple brought Apple back from the dead. It's why you have Netflix. The Fisher Space Pen, the Swiss Army Knife, and the Rolex Oyster Perpetual are some of our most enduring products. All are marvels of simplicity.