Why a Shampoo Stream Can Seem to Spring Upward
Pour shampoo onto a puddle of shampoo and the stream may curl into a loop that looks as if it has bounced back toward the bottle. The motion is an illusion of direction: the liquid keeps flowing downward, but a thin layer of the falling stream can skim across the puddle before joining it.
A Fast, Thin Stream Meets a Thick Puddle
As shampoo falls, gravity accelerates it. The stream stretches out and narrows, much like a strand of honey pulled from a spoon. That narrow stream can move quickly enough to spread over the puddle’s surface instead of plunging straight into it. Because shampoo is viscous—it resists flowing readily—the layer can persist long enough to steer the incoming liquid sideways.
The puddle acts less like a solid surface than a slow-moving path. The stream rides along that path, bending around and sometimes looping before it finally merges with the pool. From the right angle, the curve can resemble a rebound, even though no part of the liquid has reversed its fall.
The “Bounce” Is a Detour
The key is the contrast between a fast, narrow stream and a thicker, slower pool. Their flow does not immediately blend into one downward splash; instead, the stream briefly travels along the puddle’s surface. That detour is what makes an ordinary pour look startlingly springy—and why the apparent upward leap is really shampoo following the shape of its own surface.
