wh y does my pee.pee cuom e out my peepee hole?,

Assuming peepee is an incompressible, Newtonian fluid, then a rigid body, that is to say a fully turgid penis, follows the following energy conservation law:
P_in + p(V^2/2)_in + pgz_in + P_pump (in which case the pump is your bladder) = P_out + p(V^2/2)_out + pgz_out + Pf (where Pf is the amount of energy your peepee loses due to friction as it travels through your turgid penis. There is friction involved because the interior walls of your urethral tract are not perfectly smooth, so the peepee hits microscopic ridges on the wall that creates friction, that removing some energy and removing some pressure from your peepee.)

P_in refers to pressure before leaving the turgid penis, p(V^2/2)_in refers to kinetic energy in terms of pressure of the peepee before leaving the turgid penis, pgz_in refers to the potential energy in terms of pressure due to peepee height before it leaves the turgid penis, P_pump refers to the amount of pressure your bladder adds to the peepee to have it leave your turgid penis. All other terms refer to the same quantity, but are measured when the peepee leaves the turgid penis. Pf takes into account the frictional energy losses your peepee undergoes as it travels along the turgid penis and this loss is expressed in terms of pressure.)

With the energy conservation equation in place, that begs the question: why does the peepee come out? There has to be some driving force from high pressure to low pressure. We can assume negligible frictional losses on the peepee because the travel distance for the peepee would have to be hundreds of feet to be significant. Moreover, gravitational potential energy is negligible because the peepee would have to be many feet off the surface of the earth to be significant. We can simplify the energy balance further by assuming urination parallel to the Earth. In other words, we assume the penis is parallel to the Earth. If this happens to be the case, acceleration vectors perpendicular to velocity vectors do not affect the horizontal component of the velocity vector under projectile motion.

To make more sense of this, gravity would be acting perpendicular to the urine stream as it leaves the peepee hole. The peepee would accelerate in speed downwards, forming an arc of urination. However, it is important to remember that we are looking at the instantaneous moment the peepee leaves the peepee hole, thus the urine is traveling in an idealized, perfectly horizontal line. If this is true and gravity isn’t changing its horizontal speed, then the kinetic energy off the peepee right before exit and right after exit of the peepee hole should be approximately equal if not equal outright. Our energy balance simplifies to:

P_in + P_pump (aka Bladder Pressure = P_out
Which can be rearranged for,
P_out – P_in = Delta(P) = P_pump
Pumps add energy into a system by pushing fluid down a certain pathway, so the only significant increase to the peepee’s driving pressure is from the bladder because we have canceled or considered all other energy terms negligible. We can calculate the exit pressure of our peepee easily, but what about inlet pressure, or before the peepee hole? If we knew this unknown, we could know, once in for all, why our peepee comes out our peepee hole. Let’s consider the following experimental setup: