Air has weight:
- Air is made up of molecules, and these molecules have mass.
- When air is present inside a football, it adds weight to the overall mass of the football.
- The weight of the air inside the football contributes to the total weight of the blown-up football.

Comparison with a blown-out football:
- When a football is blown out or deflated, the air is released, and the football becomes empty.
- Without the air molecules inside, the weight of the football decreases.
- This is because the blown-out football only consists of the material it is made of, such as leather or synthetic materials, which are generally lighter than the air that was inside the football.

Understanding the concept of weight:
- Weight is the force exerted on an object due to gravity.
- When an object has mass, it is influenced by the gravitational pull and experiences weight.
- The weight of an object is directly proportional to its mass. The more mass an object has, the more it weighs.

Air filling space and giving shape:
- Air filling space and giving shape to things are relevant concepts, but they are not directly related to the weight difference between a blown-up and a blown-out football.
- Air fills the space inside the football, giving it a round shape and making it bouncy.
- However, these characteristics do not affect the weight of the football.

Air needed for burning:
- The presence or absence of air does not directly affect the weight of an object.
- While air is necessary for burning, it does not explain why a blown-up football is heavier than a blown-out football.

In conclusion, the reason a blown-up football is heavier than a blown-out football is because air has weight. The air molecules inside the blown-up football contribute to its total mass, while a blown-out football only consists of the lighter materials it is made of.