It seems like you're asking about the principles that govern the behavior of fluids. There are several key principles, and it's helpful to be more specific to understand what you're interested in.

Here are some of the most important principles in fluid mechanics:

1. Archimedes' Principle: This principle states that an object submerged in a fluid experiences an upward buoyant force equal to the weight of the fluid displaced by the object. This explains why objects float or sink.

2. Pascal's Principle: This principle states that pressure applied to an enclosed fluid is transmitted undiminished to every point within the fluid. This is the basis for hydraulic systems, where a small force applied to a small area can create a large force on a larger area.

3. Bernoulli's Principle: This principle relates the pressure, velocity, and height of a fluid. It states that as the speed of a fluid increases, its pressure decreases. This principle explains lift generation in airplanes and the operation of venturi meters.

4. Viscosity: This is a measure of a fluid's resistance to flow. Fluids with high viscosity (like honey) flow slowly, while fluids with low viscosity (like water) flow quickly.

5. Surface Tension: This is a force that causes a fluid to minimize its surface area. It's why water forms droplets and allows insects to walk on water.

6. Continuity Equation: This principle states that the mass flow rate of a fluid is constant in a closed system. This means that the product of the fluid's density, velocity, and cross-sectional area remains the same throughout the system.

7. Conservation of Energy: Fluid flows are governed by the conservation of energy. This principle states that energy cannot be created or destroyed, only transferred from one form to another.

Let me know if you have a specific principle in mind, and I can provide more details about it!