Fluid mechanics assumptions I have just started studying fluid mechanics and on the very first page two assumptions are made about liquids while I understood the first one but am not able to understand the Second one: 

Parts of the liquids in contact do not exert any tangential force on each other. the force by any part of the liquid on the other part is perpendicular to the surface of contact. thus there is no friction between adjacent layers of liquids. 

why are such weird assumptions being made here? what do they want to signify? Like if there is friction between adjacent layers of liquids so what? Why assume there is no friction?
 A: Suppose there is an imaginary boundary plane between two parcels of fluid, and parcel A is standing still while parcel B is moving with some speed.
Fluids have temperature and pressure, meaning they are made up of molecules in motion, constantly bouncing off each other. Suppose A and B have the same temperature and pressure, just to keep things simple
At the boundary, some of B's molecules are going to cross into A's territory, and vice-versa.
This transfers momentum, which has the effect of reducing, and eliminating, the average speed difference, at the boundary, between A and B.
So the speed difference cannot be a step discontinuity at the boundary, but is spread out through both fluids.
That's why fluids cannot slip past each other.
A: Why we can :
Such a tangential force is most important in solids, since it corresponds to the elastic response to shear. In a liquid, a sheared flow leads to a viscous force only (that's a definition of liquids, as used in seismology for example); this is usually negligible at slow - enough velocities.
Why we do :
Moreover, since shear & stress are described by tensors (as opposed to scalars/vectors) it is nice to be able to neglect them in introductory courses !
