There are two common ways to convert plants into gas and liquid fuels. One is to grow crops that are high in sugar (e.g. Sugar cane) or starch (e.g. corn), and then use yeast to ferment ethyl alcohol (ethanol). The second is to grow plants that contain high amounts of vegetable oil, such as palm oil, soybean and algae. When these oils are heated, their viscosity is reduced, and they can be burned directly in a diesel engine, or chemically processed to produce fuels such as biodiesel. For thousands of years, Wood and its by-products have been converted into biofuels such as charcoal, wood gas, methanol or ethanol fuel. It is also possible to make cellulosic ethanol from non-edible plant parts, but this can be expensive.