Foil prevents helium form escaping from the balloon so they stay filled for a longer period of time. Foil-balloons can be filled with air (fixed then to a balloon-stick or fastened with string) or with helium. Foil-balloons come in a variety of shapes and sizes. Foil-balloons are fully covered with high quality, colorful print. Foil-balloons are especially popular amongst children because balloons are often shaped as well-known characters from popular cartoons or books.