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Now we know all about carbon nanotubes. But, how are they made? To put it simply, carbon nanotubes are grown. They are grown through a process called chemical vapor deposition (or CVD for short). In CVD, a substrate is coated with a layer of metal particles (such as nickel, cobalt, or iron). Then, the substrate is heated to 700 degrees Celsius. Meanwhile, two gases are released into the CVD reactor. One gas is a process gas (such as ammonia or nitrogen) and the other gas is a carbon-containing gas (like ethanol or methane). The carbon-containing gas is broken apart by the catalyst metal particles and the carbon is taken to the edge of those particles, where it forms nanotubes. Although CVD is the most common way to produce nanotubes there are other ways as well. For instance, carbon nanotubes have been found in nature. They have been seen as products of ordinary flames, produced by burning methane, ethylene, and benzene. Carbon nanotubes have also been found in soot from both indoor and outdoor air. However, these nanotubes are nowhere near as useful as those produced by CVD. They have highly irregular size and quality and (although they can be used in some ways) are for the most part useless.