PHYSICS

Light a candle and watch the smoke rise. At first it climbs in a calm, straight thread. Then, without warning, the thread wobbles and breaks into swirling loops and tangles. That sudden change is one of the most important ideas in fluid physics. Fluids can flow in smooth, orderly layers called laminar flow, or in wild, mixing motion called turbulent flow. The same air or water can switch between the two depending on speed, viscosity, and the size of the channel. Understanding this split helps engineers design quiet pipes, stable aircraft, and efficient blood flow in the body.