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     The gas consists of identical small particles called molecules.  They are not stationary.   These molecules are very dynamic, travel in random directions with very high velocities.  They hit one other and the walls of the container.

    When the molecules hit the walls, their direction of movement changes.  The velocity becomes different.  The change of direction of the molecules, causes some force on the walls of the container.  This is like a ball hitting a wall or a plank.  When a ball hits the top of a table, we can see that the table top vibrates.

    The force exerted by all the molecules that hit the wall in random directions in a unit time results in the pressure.  Thus the pressure exerted on the ball is equal to the force / area of the walls.  If the container is big, then the area is more, so the pressure is less.    If the container is small, then the area of wall is less, the molecules hit the walls more often, so the force and the pressure are higher.

   Hence, the collisions of the molecules with the wall causes the pressure.

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