You've probably seen them on your ice cube trays, those thin spires of ice that shoot up from the surface of a cube like an inverted icicle. But how in the world did it form?
In an ice cube tray, the surface freezes first, since it's in direct contact with the cold air, from the sides of the cube in toward the center until a small hole is left in the middle, according to the California Institute of Technology.
Ice crystals start to form, and the cube freezes as it expands. The water in the middle becomes squeezed in on all sides with nowhere to go. It's forced up and out through the hole in the ice.
As the water comes up to the hole, it freezes around the edges, forming a hollow spike. As more water is forced out, the spike becomes longer. This continues until all of the water has frozen or until the end of the spike freezes shut.
In this video from Veritasium, a science channel on YouTube, Derek Muller, PhD illustrates the phenomenon:
According to the video, the holes in the ice are often triangular because ice crystals tend to meet at 60-degree angles.
An ice spike that takes the shape of an inverted pyramid is one of the rarer shapes, and this usually happens in fountains or bird baths — containers that hold more water than an ice cube tray.
When ice spikes of this size and shape occur, they're also known as ice vases, ice towers or ice candles, but the science behind them is the same.
When seen in nature, they almost look like the stalagmites found in caves.
How to make your own ice spikes
You can try to make your own ice spikes in your home freezer. Here's how, according to the Veritasium video:
1. Fill ice trays with water and put them in the freezer. The ideal temperature is between 18 and 23 degrees F. This is cold enough to freeze water but not cold enough to freeze off the tip of an ice spike.
2. Use distilled water, as that works better than tap water because even small amounts of salt prevent spike formation.
3. Put a fan in the freezer if possible; it will help spikes form by increasing air circulation and improving evaporative conditions.