An object in a fluid experiences an upward buoyant force. This force equals the weight of the fluid displaced. Whether an object floats depends on its weight and the amount of fluid it can displace. Density describes mass per unit volume. A sealed object with an average density lower than the surrounding water can float while displacing water equal to its weight. Shape matters because it affects the volume available to displace water and whether water enters the object. A metal ship can float even though solid metal is denser than water: its hull encloses a large volume containing air. This does not mean every metal shape will float. Leaks or added weight can change the balance. Keep demonstrations small and never test flotation by entering deep water. Practice: Compare a small piece of foil shaped as a ball and as a shallow boat in a tray. Describe how the shape changes the water displaced before sinking.