Why does the pressure inside a container of gas increase if more gas is added to the container?

The pressure increases because the more gas you put into the container the pressure is going to rise as well, that's because the gas is being put in the container constantly which is increasing the pressure. 

molecules are free to separate from each other and will take on the shape of the container. molecules move completely at random and high speeds. they vibrate in a fixed position and do not move around much. molecules expand to the edge of their container.