Hot Air Balloon Gifts

Fall is a beautiful time of year to take a hot air balloon ride! Hot air balloons fly by heating the air inside the “envelope” (an inner bag that is able to hold air) until it provides enough upward force to lift the basket and its occupants off the ground.

Billy’s new hot air balloon is 100 feet in diameter and looks like a basketball (it is a sphere). What volume of air, in cubic feet, will Billy’s hot air balloon hold when it is completely filled? Express your answer as a decimal to the nearest tenth.

It is NOT homework , its extra help and if someone tells me how to get the answer i would have an example !

Im helping my son answer a question for school but he doesn't always have the best comprehension and Im trying to explain it in simple terms so he (and I) get it. Here is what we wrote:

What makes a hot air balloon rise?
Warmer air rises in cooler air. A hot air balloon has a bag called the envelope that can hold hot air. Below that is a basket, this carries a source of heat which can produce a increase between the air inside the envelope and the outside air temperature. This helps to keep the balloon and it’s passengers up in the air.

Is that correct? Can you offer a better explanation? Im having trouble understanding the information on websites so please read this through and let me know if it is correct. If it isn't please suggest changes and rewrite it for me...thanks...remember he is 7, no massive words and technical terms thanks.