When one heats matter, its individual molecules vibrate more and more. These vibrators force the molecules apart, causing the substance to expand. In solids and liquids, this expansion isn't very noticeable to the naked eye, but can be measured. With gases it is aquite different. Because gas molecules aren't strongly attracted to one another, the molecules are free to move about.
Heating causes the molecules to move faster and make collisions harder.
The result is a more massive expansion. In fact, gases will increase their volume by roughly 1 percent for every 6 degrees Fahrenheit. So, hot air actually has fewer molecules in a given volume than cool air. When the balloons are on the ground, the air inside is just as dense as that outside. Using gas burners nd fans, the balloon operators force hot air into the balloon.