What is the difference between mass and weight?
Mass versus weight
A common helium-filled toy balloon is something familiar to many. When such a balloon is fully filled with helium, it has buoyancy—a force that opposes gravity. When a toy balloon becomes partially deflated, it will often become neutrally buoyant and can float about the house a meter or two off the floor. In such a state, there are moments when the balloon is neither rising nor falling and—in the sense that a scale placed under it will have no force applied to it—is, in a sense perfectly weightless (actually as noted below, weight has merely been redistributed along the Earth's surface so it cannot be measured). Though the rubber comprising the balloon has a mass of only a few grams, which might be almost unnoticeable, the rubber still retains all its mass when inflated.