I'm sure this is a totally amateur question, but today I got one of those big ice cube trays for 2x2x2 inch cubes. I filled it up and put it in my freezer, and about 6 hours later checked on it but the water was not frozen at all. Not even a crust on top. I put my finger in one of the squares and swirled it and in that moment the purely liquid water became a snow-like consistency. I did it again with the other squares and the same thing happened. What causes this?