Where would cities be without asphalt? It is a wonder material for building roads and parking lots because it is cheap, easy to work with, smooth, seamless, and durable. Engineers can spec out an asphalt road that can handle millions of cars for a decade or more. Asphalt is so popular that all but 6 percent of America's roads are made of the material. One of the earliest known uses of asphalt goes back to 625 cE in Babylon. One reason for asphalt's popularity is its simplicity. Asphalt has three basic components: sand, gravel, and bitumen. Although found occasionally in nature, bitumen today comes from refineries. It is separated out from crude oil in the same way as other petroleum distillates. Gasoline and bitumen contain the same atoms (carbon and hydrogen), but the carbon chains are immensely long in bitumen. Therefore bitumen is approximately a solid at room temperature. It is possible to make asphalt by hand. You could even make a small amount in your kitchen oven. Put some sand and gravel on a cookie sheet. Put it in a 300°F (150°C) oven long enough for it to heat evenly and dry out. Now place a lump of bitumen on it and continue heating until the bitumen melts. Stir thoroughly to mix bitumen and gravel together. You have asphalt. Your kitchen will stink and you will never get that cookie sheet clean, but now you can fix a small pothole. Engineers have refined the creation of asphalt and the equipment that handles it to push down the cost of roads. For example, the Strategic Highway Research Program created mixing and construction guidelines for SuperPave, the asphalt recipe used in many highways today. On a big road project, engineers will often erect a portable asphalt plant near the construction site to make delivery easier and more consistent. One little-known fact about asphalt is that it is the most recycled material, by weight, in the US. Construction crews grind it out and re-add bitumen to make new roads. If it weren't for asphalt, engineers would need to use concrete, which is much more expensive. This makes asphalt one of the most popular construction materials in the world. >The process for the creation of asphalt has been refined since its first use in Babylon in 625 BCE, when Herodotus recorded it being used as mortar.