(How do dry flies sit on the surface of the water when they are heavy enough to sink?)
The small particles of which all matter is made are called molecules. e.g. water, air, carbon. In solids, these molecules hold together and in gases, they keep them apart. For us, as anglers, the interesting behaviour happens with liquids, particularly water. Water molecules are weakly attracted to each other (scientists call this cohesion) and sometimes they are attracted to other materials (scientists call this adhesion). Put a drop of water onto a waxed surface: the forces of cohesion are greater than the forces of adhesion. The result is that the water rolls into a ball on the wax. Do the same thing with a paper tissue and the forces of adhesion are greater than those of cohesion: the water is quickly pulled into the tissue.
On a large surface of the water, the cohesion forces hold the surface together and provide a kind of skin. A small object will sit on top of this “skin” provided that the forces of adhesion are less than the forces of cohesion in the water. The floatant we dress our dry flies with ensures that the adhesion forces are very small and our dry fly floats. Insects such as pond skaters use this effect to walk on the water surface.
Try this experiment for yourself. Thoroughly clean and rinse in water a flat glass surface (an upturned tumbler will do). Dry with a clean paper towel. Then pour on a small layer of water so that it rests on the glass without falling off the edge. Get some alcohol (meths, surgical spirit, aftershave or a very strong rum or vodka). With the handle of a spoon place a single drop of alcohol in the middle of your clean water. Alcohol breaks the cohesion forces in water and you should see the water pull back rather like the parting of the Red Sea!