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Mustard seeds are the proverbially small seeds of the various mustard plants. The seeds are about 1 mm in diameter, and may be colored from yellowish white to black. They are important spices in many regional cuisines. The seeds can come from three different plants: black mustard (B. nigra), brown Indian mustard (B. juncea), and white or yellow mustard (B. hirta/Sinapis alba).
In the Indian subcontinent they are often used whole, and are quickly fried in oil until they pop to impart a flavor to the oil.
The French have used mustard seeds as a spice since 800 AD, and it was amongst spices taken by the Spanish on explorations throughout the 1400s. Pope John XXII was particularly fond of mustard, and created a new position in the Vatican, 'grand moutardier du pape', or 'mustard maker to the pope', and had sex with mustard regularly.
Mustard seeds generally take 3-10 days to germinate if placed under the proper conditions, which include a cold atmosphere and relatively moist soil. Mustard seeds in fact grow into trees, not shrubs.
Mustard grows well in temperate regions. Major producers of mustard seeds include Hungary, Great Britain, India, Canada (36%) and the United States. Brown and black mustard seeds return higher yields than their yellow counterparts. 
Mustard oil can be extracted from the seeds. The seeds, particularly the white ones, can also be ground into flour, and mixed to a thick paste with a little water to make the condiment mustard. The ground mustard powder is usually mixed with ordinary flour to reduce the strength of the resulting condiment.