How a Smooth Water Thread Starts to Break
A faucet stream can look steady as it falls, but it is also a thin column of water being pulled downward by gravity. Tiny unevennesses—perhaps from the faucet opening, vibration, or air moving around the stream—make one part slightly wider and another slightly narrower.
Surface tension, the pull created by water molecules attracting one another at the surface, acts to reduce the stream’s exposed surface. It therefore favors growth of the wider bulges and shrinking of the narrow necks between them. As the necks thin, the moving water can no longer keep the column connected, and a neck pinches through. The process is called the Plateau–Rayleigh instability: a long, narrow liquid thread naturally tends to split into separate drops.
Why the Pieces Become Round
Once a section has separated, surface tension pulls it toward a compact shape. For a given amount of water, a sphere has less surface area than an elongated shape, so a free drop tends to round out. Falling drops may wobble or stretch briefly, but the same surface pull keeps reshaping them.
The stream does not necessarily break at the faucet itself. It can travel some distance before small disturbances have grown enough to make a neck snap. The faucet’s flow rate and opening affect how thick and fast the stream is, while gravity and those disturbances influence where breakup occurs.
That is why a faucet may deliver an apparently continuous ribbon close to its spout and a chain of drops farther down: the water is flowing all along, but surface tension gradually turns an unstable thread into separate, rounder pieces.
