indicate the particle that was at 
,
, 
at time t=0.
In a Lagrangian description, we want to find the motion of the fluid particles (little regions of fluid).
We must first label the fluid particles with tags. For example, let
indicate the particle that was at ,, at time t=0.
We will want to find the position
of every particle in time.
In other words, we want to find 
. Then
Example:
2D ideal stagnation point flow
Shape of the particle paths:
is constant.
