* Lower air pressure: As you go higher in altitude, the air pressure decreases. This is because there's less air above you pushing down. Think of it like a column of air, the higher you go, the less air is in the column.
* Reduced density: Lower air pressure means the air molecules are spread further apart, making the air less dense. This results in fewer oxygen molecules per unit of volume.
* Gravity: Gravity pulls the denser air towards the Earth's surface. As you go up, the air gets thinner because gravity's pull is less strong.
Think of it like this: Imagine a balloon full of air. If you take the balloon to the top of a mountain, the air inside the balloon would expand because the air pressure outside is lower. The same thing happens with the air in the atmosphere.
This is why people can experience altitude sickness when they climb high mountains. Their bodies have to work harder to get enough oxygen because there's less of it available.
