Imagine you decide to start riding your bike to work instead of driving your car. You want to cut down on emissions, use less gas, and get more exercise. You purchase a new helmet. You fill your tires with air. You roll up your pant legs and sling your bag over your shoulder. Off you go, feeling great about getting this extra workout in, getting to work on time, and getting to help the health of the planet in some small way. You pull up to a stoplight, and while you’re waiting for it to turn green, a man walks past and yells at you, “Those tires are made of…