I want to test the ability of students to read a passage and answer questions about it. Could you please come up with a good question for the passage "A solar balloon is a black balloon that is filled with ordinary air. As sunlight shines on the balloon, the air inside is heated and expands causing an upward buoyancy force, much like an artificially heated hot air balloon. Some solar balloons are large enough for human flight, but usage is generally limited to the toy market as the surface-area to payload-weight ratio is relatively high."? 

What is an important aspect of a solar balloon to function?