Mustard is one of the world's oldest condiments, with the Romans being the first to grind the seeds into a paste. The 'heat' of mustard is not caused by the seeds themselves but by a chemical reaction. Mustard seeds contain a compound called glucosinolate and an enzyme called myrosinase. When the seeds are crushed and mixed with cold water, the enzyme triggers the production of allyl isothiocyanate (mustard oil). This is a defense mechanism for the plant to deter insects. The intensity of the flavor can be controlled by the temperature of the liquid; cold water creates a fiery heat, while hot water or acid (like vinegar) denatures the enzyme, resulting in a milder mustard. English mustard is traditionally mixed with cold water, which is why it is much hotter than French or American varieties.