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Helium balloons work due to buoyancy, an upward force exerted by air on the balloon. The balloons are filled with helium because helium is a very light gas, lighter than oxygen. Since it is lighter than the air around us, it can float. However, a house, like the house in the film Up, could probably never float due to this buoyant force of helium balloons - some estimates say that you would need 23.5 million helium balloons to get that house off the ground! In the film, animators drew about 20,000 balloons, which wouldn't be enough to lift a house.