When you put ice in a glass of water, it does not sink. It floats on the top. This looks normal, but it is a special property of water.
When water freezes, its molecules form a fixed pattern. This pattern has open spaces, so ice takes up more space than the same amount of liquid water. In other words, ice has lower density than liquid water.
Things with lower density can float on liquids with higher density. That is why ice floats on water. This also matters in nature. Ice forms on the top of lakes, helping the water below stay liquid.