How come the filled water bottle work as a stylus for phone but not the empty bottle? Take an empty plastic bottle, and make it slide on a mobile screen. You would find that it's not doing anything. Now filled up this bottle with water and try doing the same. This time you will find that it's able to make the screen slide.

I wanted to know, How come filling up the bottle with water makes the difference?
I was seeing the working of the touch screens, Most of the mobile screens are resistive or capacitive type. The resistive screen is based on pressure response so it should make no difference whether or not the bottle is filled or not. On the other hand, the capacitive based on the charge that flow through the bottle to screen, The bottle is an insulator, so the charge is hard to flow. If you filled up the bottle, still there is a plastic in between the water and the screen, So it should not work again but it does. How come?
 A: This is on the assumption that you have not accidentally poured water on the outer surface of the bottle (no matter how little) - i.e., this surface is perfectly dry:
Explanation 1:
Note that air (the atmosphere around us) contains water molecules as well.
When you put cold water into a plastic bottle, due to condensation$^1$, water will collect on the outside of the bottle$^2$. Since water contains ions (and is therefore conductive), a capacitive touchscreen will react to such a surface in contact with it.

*

*Water collects as droplets on a cooler surface when warmer air is in contact with it. In this instance, the outer surface of the filled bottle becomes cooler then the surroundings (which is assumed to be warmer i.e., you're not in a cool environment, but for example, you are inside your home).


*I know that maybe not much water will collect when using standard tap water (because it is not that cold), but because the water is pretty much cooler than room temperature, some condensation will occur and my understanding is that some of these capacitive screens are pretty sensitive.
Capacitive touch screens react when any conducting surface (like your skin) comes in contact with it.
Explanation 2:
It is well known that a bottle filled with salt water can behave like a capacitor since salt water is an electrical conductor.
It is also known that tap water does contain charged particles, so it may be the case that a bottle of water can also behave (albeit much less pronounced than with salt water) as a capacitor.
How to test if both the above explanations are valid:
Redo your experiment, this time:

*

*Use warmer water to test the validity of the the first explanation.


*Try using distilled (pure) water to test the validity of the second explanation (distilled water has no conductive properties). If available, dip the two ends of a multimeter in the distilled water to indeed confirm it contains no anions/cations.
A: *

*The difference between an empty bottle and a bottle filled with water is the difference between a resistor and a capacitor (or condenser).


*A capacitor does not let a continuous current through, but it does let a time-varying current through: https://www.allaboutcircuits.com/textbook/direct-current/chpt-16/capacitor-transient-response/.
On a historical note: The first capacitors were built with bottles and jars (see the Leyden jar):
https://en.m.wikipedia.org/wiki/Leyden_jar
A: I don't know technical details but if the phone can detect the presence of a mostly water thing nearby (such as your finger) then it might be doing so by detecting a change in either capacitive or inductive properties near the screen.
Any piece of conductor (such as an element of the screen) can make one half of a capacitor, the other half being the ground. Then if you change the medium between the parts of this capacitor from air to water, then the capacitance will change. A change in capacitance can be detected, for example, by applying an oscillating voltage to the capacitor and finding out how much oscillating current there is. Similar statements go for inductors. The oscillation frequency might be some kHz or MHz I guess.
