Golf Balls with Dimples
When golf first became a popular sport, the balls were perfectly smooth. However, balls with dimples allow for a layer of turbulent air to form around its surface, which shrinks the size of the ball's wake when it's hit.
The Arrow Next to the Gas Gauge
Have you ever driven someone else's car and then realized you have no idea which side the gas tank is on when you pull up to the station? That arrow on the gas gauge actually points to which side the tank is on. That way, you don't have to waste time checking and moving your car.