The velocity of a fluid is zero at a stationary boundary due to the no-slip condition. This condition states that there is no relative motion between a fluid and a solid surface in contact with it. In other words, the fluid sticks to the surface and moves with it.

The no-slip condition is a consequence of the viscosity of fluids. Viscosity is the property of a fluid that resists the flow of the fluid. When a fluid flows over a surface, the viscous forces between the fluid and the surface exert a drag force on the fluid, which slows it down until it eventually comes to a stop.

The no-slip condition is important in many applications, such as the design of aircraft wings and the flow of fluids through pipes. It is also important in understanding the behavior of fluids in natural systems, such as the flow of water in rivers and oceans.

Here are some examples of the no-slip condition in action:

* When you stir a cup of coffee, the coffee at the bottom of the cup does not move as fast as the coffee at the top. This is because the coffee at the bottom is in contact with the bottom of the cup, which is stationary.

* When you blow air over a piece of paper, the paper does not move. This is because the air is in contact with the paper, which is stationary.

* When you swim in a pool, the water at the bottom of the pool does not move as fast as the water at the top. This is because the water at the bottom is in contact with the bottom of the pool, which is stationary.

The no-slip condition is a fundamental property of fluids that has important implications for many applications.