Ask a Physicist: Conservation of Mass Violation...In a Bowl of Couscous?

Cal, from Italy, wants to know: "When I add hot water to couscous in a bowl, and then zero out the scale it sits on......it magically starts increasing in weight over time as it absorbs the hot water!I can understand it increasing in volume, but not in weight. How does this happen?" Cal,I love questions like this! It's like a puzzle, where sometimes there's an opportunity to use physics and logic to peer into the inner workings of things and figure out a solution from thousands of miles away. It...