Ah, brings back fond memories. I always loved physics simulations and managed to implement a fluid solver (after reading [1]) back in 2005. When I got the basic 2D version working, I spent maybe 20 minutes just stirring the vortices with a mouse - so fun!

[1] Visual Simulation of Smoke (physbam.stanford.edu/~fedkiw/papers/stanford2001-01.pdf)