For a simple example, let’s say you know the tires on your car are worn to the point where there is very little tread left. Because money is tight, you decide to defer purchasing new tires for a few more months. At that point every trip you take in the car carries with it the increased risk that you will skid if you have to stop quickly. A few weeks later it is raining hard as you hop in the car to do an important errand. Even with new tires, the wet roads increase the risk that you will skid, especially if you have to stop suddenly. However, with bald tires that risk is increased to the point that, if someone pulls out in front of you, you will not be able to stop in time and will find yourself staring at your crumpled wreck of a car, kicking yourself for not getting the new tires. The combined risk of the bald tires and the wet roads was much greater than the sum of the two separate risk factors.