To predict how many water drops can you put over a coin before it overflows.
a clean coin, an ink dropper
Place the coin perfectly horizontal. Start putting drops of water on the coin slowly with the help of a dropper. Keep counting the drops. Guess it but rely on your findings. To your surprise, the coin hold many more drops then you initially guessed.
Surface molecules of water form a thin film. For small surfaces, this film keeps the water intact.
Observe that water surface become convex as you put more and more drops on it. This convex surface protrude out of the coin but does not fall. When too much water protrude out, the film is unable to hold any more, it breaks and water overflows.
Detergent weakens the surface film. Dip the coin in soap water and see how many drops it accommodates now. Observe the shape of the drops. On a clean and dry coin a drop is nearly spherical but it flattens on a soapy coin.