A student injects a thin stream of dye into water flowing through a glass tube. At low tap opening the dye forms a straight line; when the tap is opened wide, the dye mixes immediately across the tube. Explain. Step 1: At low speed the flow is streamline (laminar): each water particle follows the same path, so the dye stays as one unbroken thread along a streamline. Step 2: Opening the tap raises the speed past the critical value (Reynolds number exceeds ~2000), and the flow becomes turbulent. Step 3: In turbulent flow, eddies fling dye particles across the tube, mixing it instantly. Answer: The straight thread shows laminar flow; sudden mixing shows the transition to turbulence. This is Osborne Reynolds' famous 1883 experiment.