A balloon filler is a substance used to fill balloons, and the most common solutions are HELIUM (6 letters), WATER (5 letters), GAS (3 letters), AIR (3 letters), and hotair (6 letters). HELIUM is a non-flammable, colorless, odorless, and tasteless GAS that is lighter than AIR, making it ideal for filling balloons. WATER is also a popular choice for filling balloons, as it is easy to obtain and relatively inexpensive. GAS is another option, although it is not as commonly used as HELIUM or WATER. AIR is also an option, although it is not as effective as HELIUM or WATER in terms of buoyancy. Finally, hotair is a less common option, but it can be used to fill balloons with a greater degree of buoyancy than AIR.