The idea of divergence and curl*
http://www.math.umn.edu/~nykamp/m2374/readings/divcurl/ this web site is so unbelievable in its effort to make the concept of curl of a vector field understood. The 3-D image of curl and div of a vector field is clearly explained and shown with 3-d image.
This is much better than those image on the flat 2-D [...]
