Water turns into ice when the temperature drops below 0 degrees Celsius (or freezing point) but what actually happens? Water is made up of miniscule pieces called molecules which are too small to be seen, even under a microscope. These molecules move around all the time while the temperature is above freezing. Once things start to cool, the molecules slow right down. That's when liquid water becomes solid ice.
Water expands when it freezes but although the ice takes up more room than water, it's actually lighter. As ice weighs less than water, it will float. In winter, the floating ice on a pond or lake traps warmer water underneath. Fish and other creatures, such as hibernation frogs, buried in the mud at the bottom, can survive the cold weather.
After a freezing cold night of frost and ice, you might notice that some plants that have been growing in your garden during the summer, such as brightly-coloured nasturtiums, will turn brown or just flop over. These are usually plants that come from warmer countries and cannot survive our winter temperatures. Plant cells in the leave and stems are full of water, so when this freezes and expands, the delicate cell walls break down and the plant dies.
Plants that normally live in colder climates are much tougher, and they have a clever way of surviving the winter. They produce lots of sugar, which stops the water in the cells from freezing. In fact, people who grow brussels sprouts and parsnips like the frost because it makes the plants produce sugar so they taste sweeter. Some seeds and plant like hawthorn and apple trees actually need to go through a spell of icy weather before they can start to grow or germinate. This is called dormancy and it protects the seeds so they don't start growing before the warmer days of spring. If you want to grow these seeds indoors in a warm house, you have to put them in the fridge for a few weeks first so they think it's winter!
Experiment - you can see for yourself how ice breaks down plant cells: out a small, firm, fresh tomato in the freezer and leave it there for a few days. Then get it out and let it thaw. you'll see that it's now gone soft and squidgy. This is because when the water inside the tomato froze and expanded it burst the cell walls that kept the tomato firm. You can still eat it though, it will taste just fine in pasta sauce or soup.