spikes can be formed artificially on ice cubes produced in domestic refrigerators; using distilled water in plastic ice cube trays. The formation of the spike is similar to that of the naturally occurring spike in that the expansion of interior water and, the reduction of volume in the interior of the cube, increase the pressure on the water pushing it upward through the hole. The growth of the tube ceases when the drop at the top of the tube freezes entirely, which is substantially before the rest of the water in the cube is frozen. This method produces small spikes which are usually round or triangular in cross section with sharp tips. Experiments using this method have been carried out in laboratory settings but it has been found that spikes are less likely to form in ice cubes made from non-distilled water as impurities in the water inhibit spike formation. This poses the question of how naturally occurring ice spikes form in tapwater or rainwater and, Libbrecht and Lui have suggested that, in the case of the small spikes grown in a refrigerator, impurities will become increasingly concentrated in the small unfrozen droplet at the top of the tube reducing the freezing rate and so the growth of the tube. However, they believe that on the rare occasions when exceptionally large spikes grow in natural, outdoor ice formations, some other mechanism must remove the impurities that build up at the top of the growing tube. Either impurities are forced into pockets that freeze more slowly, or perhaps a convective flow, which would be insignificant in the smaller, artificially grown spikes, replaces the water at the top of the tube with fresh water from below. The results of the work carried out at Caltech have suggested experiments that might further elucidate this phenomenon. See also Candle ice Crystal growth Icicle Penitente (snow formation) Explanatory notes References Further reading External links Articles containing video clips Snow or ice weather phenomena Water ice