I recently saw this picture posted on Twitter which shows a so-called ice spike rising from an ice cube tray.
I have read the Wikipedia page, but it doesn't mean much to me. My instinct was that it is caused by vibrations from the freezer setting up a resonant frequency on the surface of the water, and the spike has gradually "built up" as the water freezes. But I'm not a physicist, so I don't know if this makes sense. I'm ideally looking for a layman's explanation of why this happens.
Answer
I think what is happening in rough qualitative terms is that the water freezes around the sides and the top first leaving a hole in the centre. Ice expands by 4%-9% when freezing so as the water below freezes it forces the remaining water up through the hole where is freezes around the edge. The hole shrinks as the water freezes and rises around its edge forming the base of the spike. The spike is hollow so water is pushed up to the top where it freezes at the edge making the spoke grow longer
Update: If you search for ice spike on youtube you will find some good timelapse videos showing these spikes forming. I especially like this one because you can see the unfrozen water pushing up inside the spike.
Sometimes in a larger water container you can get an inverted pyramid shape. The explanation is the same and the shape is due to the crystaline nature of the ice. This video shows the phenomena
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