Notice the characteristic ripples on the icicle--see Ripples on Icicles for videos showing how these ripples form. Visit Geomefreeze to find out more about icicles. To learn about the physics of freezing and melting water, see the Hyperphysics page Phase Changes.
(This feature was updated on July 17, 2013.)
credit: Stephen Morris, The Experimental Nonlinear Group, Dept. of Physics, University of Toronto; image source; larger image