Preschoolers will be intrigued by the idea that water turns into ice. Allow them to play with the ice to familiarize themselves with its properties, and then have them experiment to see whether ice melts faster in hot water or cold water, in the sun or in the shade. They can also draw with ice on a hot sidewalk, and see the water eventually evaporate.
If it is a really cold day with temperatures below freezing, you can do a fun experiment with older children to demonstrate the Mpemba effect, where hot water freezes faster than cold water under certain conditions. Have them hypothesize whether cold water or hot water will turn to ice faster if you throw it outside, then throw a cup of cold tap water and a cup of boiling tap water into the air outside. They will be amazed when the hot water turns to ice before it hits the ground but the cold water doesn't freeze until after it has been on the ground for a short time. This is because the hot water is less viscous and breaks into smaller drops of water than the cold water, giving it more surface area in contact with the freezing air and allowing it to freeze more quickly.
Older children and teenagers can experiment further with the Mpemba effect, comparing the freezing times of distilled and tap water. This experiment involves four water samples: hot distilled water, hot tap water, cold distilled water and cold tap water. Make sure the hot water is at least 100 degrees Celsius and the cold water is 25 degrees Celsius or colder. The hot tap water will freeze faster than the cold distilled water due to the different mineral composition of the two waters.
More advanced students can research the Mpemba effect and try to cause this effect by altering other characteristics of the samples used in their experiments. Changing the sizes and shapes of the containers, the type of water, and the freezing method are some ways to investigate further the conditions that cause the Mpemba effect.