When the speed of a fluid flowing in a horizontal pipe increases, the internal pressure decreases. This is due to the principle of conservation of energy and the Bernoulli's principle.

Explanation:

* Conservation of Energy: The total energy of the fluid remains constant. This energy is comprised of potential energy (due to height), kinetic energy (due to velocity), and pressure energy.

* Bernoulli's Principle: This principle states that as the speed of a fluid increases, its pressure decreases. In a horizontal pipe, the potential energy remains constant. Therefore, an increase in kinetic energy (velocity) must be accompanied by a decrease in pressure energy.

In simpler terms:

Imagine the fluid particles as little balls moving through the pipe. When they speed up, they have less time to push against the walls of the pipe, resulting in lower pressure.

Important Note: This applies to ideal fluids (inviscid, incompressible). In real-world scenarios, factors like friction and viscosity may influence the pressure change.