The photos above show how a drop of water freezes on a cold surface: by forming a sharp point at the top:
Researchers at the University of Twente, in the Netherlands, placed water droplets on a plate chilled to -20 degrees Celsius and captured images as a freezing front traveled up the droplet.
The photos are published in the American Institute of Physics' (AIP) journal Physics of Fluids. The approximately 4-millimeter diameter droplets took about 20 seconds to freeze. During the final stage of freezing, the ice drop developed a pointy tip, as can be seen in Figure 1d. The effect, which is not observed for most other liquids, arises because water expands as it freezes. The vertical expansion of the ice, in combination with the confining effect of surface tension on the spherical cap of remaining liquid, leads to the point formation.
But that's not all! After the water has frozen, the sharp tip of the ice attracts water vapors in the air and grows a "tree" of ice crystals!