A water parcel enters in it at time \(t_{in}\) at position \(\vec{x}_{in}\) and exits at time \(t_{ex}\) and position \(\vec{x}_{ex}\). Its travel time is, by definition \(T_{ }:=t_{ex\ }-t_{in}\). Let us define \(s\left(t,\ \vec{x};t_{in},\vec{x}_{in}\right)\) the volume (mass or moles, at convenience) of water parcels at position \(\vec{x}\) at time \(t\). A little daemon, She, collects all the parcels inside \(a\ neighborhood\ of\ x\ \in\Omega\) (not in its boundary) at time time \(t\) obtaining