Mustard seeds are tiny, and it takes about 500,000 of them to make a kilo. In the raw, they have taste but are not spicy in the mouth. Crushed, they become pungent following a chemical process of synaptic fermentation by a liquid (verjuice, vinegar, white wine). Mustard seeds contain sinigrin that, hydrolyzed on contact with liquid by an enzyme - myrosynase - reveals allylsenevol i.e the essential oil responsible for the spiciness of mustard.