In this experiment, a number of coins are put into a cup full of water, without spilling it.
Firstly, let me clarify one thing.
If you fill up a cup of water to the brim, in such a way that even another drop of water will cause it to overflow, can this cup take a coin, instead of a water-drop?
- If no, then it doesn't matter and there's no point to this question.
- If yes, then my question is this: why another coin and not another water-drop?
---This part exists only if the answer to the question above is yes---
They say it's because of surface tension, but I still don't get how that explains it. If the surface tension can hold together another coin, then why not another drop (which actually has less volume than a coin)?
The only reason I can think of is that the adhesion between the water and the coin is high, which sorta pulls the water molecules towards the coin, increasing the density of water immediately around the coin - thereby making up for the extra space the coin takes.
(But thinking about that, that doesn't seem to make much sense either. The coin is at the bottom, and I don't know if adhesive forces can produce such increases in density.)
So: what is the reason for this? Why a coin and not another water-drop?