The state of the air in the balloon is the key to the problem. In order to understand where the lift comes from, we must first ask ourselves some simple questions:

• Is the temperature inside the balloon greater than, less than or equal to the outside temperature?
• Is the air pressure in the balloon greater than, less than or equal to the air pressure outside?
• Is the number of molecules of air per unit volume inside the balloon greater than, less than or equal to the number of molecules per unit volume outside the balloon?