👉 At its core, a fluid is a substance that can flow and take the shape of its container. This means fluids have no fixed shape but can adapt to fill any space, whether it's a balloon, a liquid in a container, or even the air we breathe. They can move freely and are characterized by their viscosity, which is a measure of a fluid's resistance to flow. Examples include water, oil, and air, all of which exhibit fluid behavior by flowing and conforming to their surroundings.