If you have poured tea from a mug you’ll intuitively know what a Coanda effect is. To put it simply, the Coanda effect is the phenomenon where fluids like water tend to follow and stick to a contour of an object.
So what happens here? When the water flows out of the mug, the water molecules encounter the air molecules and try to drag them along due to viscosity. As the air molecules under the mug get dragged off, the pressure at that spot, which is relatively constrained compared to the other side, decreases (Bernoulli’s principle). And as the pressure is higher at the top of the water than on any other side, the water reaches equilibrium by moving towards the low-pressure region, which is what makes it to stick to the surface of the mug.
Another possible explanation is surface tension and molecular attraction. Similar effect can be seen if you hold the curved side of the spoon under a tap water.
Simple explanation. Nice! I like to add this. Coanda effect cannot happen in a container with a sharp lip. The sharp edge minimizes the surface area where viscous forces become negligible. The lips can be tapered sideways too. It keeps the flow separated from the boundary. This ensures that gravity is the significant force that is pulling the flow down and not viscosity.