Why did my chocolate seize in the bowl?

Chocolate seizes (turns grainy and stiff) when a tiny amount of water contacts melted chocolate — even steam from a poorly sealed bain-marie can cause it. Fat (cocoa butter) and water normally separate, but a small amount of water binds to sugar and creates a thick paste. Paradoxically, adding more water (2+ tablespoons per 100g of chocolate) and stirring vigorously can rescue seized chocolate into a ganache, suitable for a sauce or truffle filling but not for molding.