When they are first ground , and then mixed with water , vinegar and other ingredients , a condiment called mustard can be made . The seeds can also be pressed , to give a kind of oil called mustard oil . Mustards are several plant species in the genera Brassica and Sinapis whose small mustard seeds are used as a spice and , by grinding and mixing them with water , vinegar or other liquids , are turned into the condiment known as mustard or prepared mustard .