Elements do not usually exist on their own. In the natural world, they are found in combination with other elements. By understanding how elements combine, scientists have been able to make new combinations, creating molecules that are not found in nature. These combinations are not made simply by mixing two or more substances together. Brown sugar and salt can be stirred together, for example, but this does not create a new substance. Each little particle is either a grain of sugar or a grain of salt — they have remained separate. Mixtures can usually be separated again, but when elements are chemically joined together, they are said to be bonded and have created a new substance.
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