Essential oils often contain monoterpenes such as alpha-pinene and limonene, with variation by species and origin. Burning resin produces a different odour from smelling the unburnt oil. Distillation favours the volatile fraction, which helps explain the brighter character of many frankincense oils. Resin extracts retain a different balance of aromatic material, while smoky interpretations may require additional odorants to suggest the smell of heated incense.