Proving the existence and uniqueness of a global solution for 3D Navier-Stokes Equations (NSEs) is one of the Clay Mathematical Institute’s Millennium Problems which hasn’t been solved yet. In this talk we will discuss that what does it means to have a solution of a PDE and what is it that fails in the case of a 3D NSEs and works for a 2D NSEs. If the time permits I will give a brief description about my current research related to NSEs.