Try this experiment (outside!).
Fill a drinking glass to overflowing with water, and place a sheet of cardboard over the top. Now carefully tilt the glass in various directions. You should even be able to hold it upside down.
• Why does the water stay in the glass?
With care, with the glass in the upside-down position, you may find that you can even remove the cardboard, leaving the water in place.
This experiment shows that air has pressure, and that this pressure is exerted in all directions.
