What if you could somehow supercool the water? That is, say that you could get the water's temperature to 10 degrees below the freezing point without it crystallizing into a solid -- you can sometimes do this using a very clean glass and distilled water so there are no points for the water to begin crystallizing. In this condition, if you tap the glass the temperature of the water will jump up to 32 degrees F (0 degrees C), and the water will solidify quickly.