if we fill water in a cap of a pen and then keep it straight , why does the water did not fall from it by keeping straight and when we move it for two times why it fall?
Answers
Air pressure.
The water is trying to get up into the bottle from underneath, which means that the air inside has nowhere to go. As water tries to push into the bottle, it compresses the air, causing the air pressure to rise, which pushes back against the water.
By the way, it isn't exactly true that water doesn't go into the bottle, a little bit of water gets into the mouth. And the deeper you take the bottle, the more it goes inside. If you were to take it to the bottom of a swimming pool, for example, you'd notice the water started to creep up inside the bottle. That's because there's more water pressure deeper down, and that pressure compresses the air more and more, making room for the water to get further and further inside. But, as long as the air has no way to escape, it's still going to be there, and will always take up some volume.
Now, if you gave the air a way out, like poking a hole in the bottle, or running a straw or tube into it from the outside, you'd see the water level rise as it pushed the air out.