pressure:  Pressure is the action of a continuous force over an area.  Pressure can be thought of as a constant squeezing or pushing.  Solids, liquids, and gases all exert pressure.  When you are underwater, you can feel the effects of increased pressure.  It's the weight of all the water above you pushing down on your body.  Even the air exerts pressure.  The weight of Earth's atmosphere pushes down on everything.  However, the human body has adapted to the amount of atmospheric pressure, and so it isn't noticed.  Very high pressure can change a gas to a liquid and a liquid to a solid.  Pressure at the core of a planet is extremely high.  It is result of the combined weight of the atmosphere, the crust, and the mantle.