People who live in the Washington area know a lot about hot weather. But we get cold weather here, too. Sometimes, it gets so cold that the water in the reflecting pool in front of the United States Capitol freezes hard and thick enough for people to skate on it. Sometimes, the Park Service even lets people go ice skating on the canal beside the Potomac River!

But ice that's thick enough to skate on one day can grow dangerously thin or turn into slushy mush another day.

Why does ice change so much? Because it depends on the temperature -- and the temperature can change very quickly.

As anyone who has ever made ice cubes knows, ice is the solid form of water. Water can take three forms: a liquid, a solid and a gas (water vapor or steam). When water freezes, it forms hexagonal, or six-sided shapes. These crystals lock together to form solid ice. Frost, snow, sleet and hail are all forms of ice. So are icebergs -- those immense pieces of frozen seawater that float in the ocean. So are glaciers, the huge masses of ice that creep down valleys in high mountain ranges.

Ice begins to form when the temperature reaches the freezing point of 32 degrees Fahrenheit (0 degrees Centigrade). As it gets near the freezing point, water expands, or takes up more space. That's why ice cubes take up more room in an ice-cube tray than the water you poured in to make them did. Oddly enough, the expansion makes solid ice lighter than liquid water. That's why ice cubes -- and even icebergs that weigh many, many tons -- float in water.

When the temperature hits 32 degrees, shallow puddles begin to freeze. The bird feeder in the backyard will get a thin skin of ice across it. This new ice is so fragile that the sun's rays can easily melt it away. You wouldn't want to go skating on a pond covered with ice that thin.

If temperatures stay low, ice layers gradually grow thicker and thicker. But when temperatures rise, the ice melts back into its liquid form again.

Safety experts know that ice is unpredictable. It can look like a solid mass but really have different thicknesses. It can contain cracks or even open places that are hard to see. Ice over still, shallow water -- like the Capitol's reflecting pool -- is usually the safest.

The thickness and stability of ice depends on more than temperature. Winds can ripple water enough so that it freezes at a different rate than still water. Moving currents also slow down ice formation. Even the currents set off by fish swimming under the surface can affect the formation of ice. Variations in temperature in the water in a lake or pond affect the rate at which the ice freezes and grows thick.

Ice also gets weaker as the sun's warmth builds up during the day and if the weather gets warmer.

To freeze a deep lake or pond thick enough for safe skating takes about two weeks of overnight temperatures near zero degrees, experts say. And that's zero Fahrenheit, not zero Centigrade!

But even in very cold parts of the country, like Minnesota, a solid-looking pond or lake can be deceiving. In some places, the ice could be 10 inches thick, but in other places it might be just one inch thick. Safety experts say ice should be at least 4 inches thick before people attempt to walk or skate on it.

That's why it's a bad idea to trust ice yourself. If you're on a walk in the country and you see an inviting-looking frozen pond, don't go sliding on it. You could be risking your life.

Even if you go skating on a pond that adults have decided is safe, you should be careful. Don't go way out to the center; the ice there is thinner than the ice near the banks. Safety experts also warn against skating on ice that's above deep water. Even if you're a terrific swimmer, you could be in serious trouble if you fell through. The shock of the cold water can paralyze your muscles in just a few minutes.

If you ever see someone fall through ice, stay off the ice yourself. Call for help immediately. Rescuers will slide a ladder or a stick with a rope attached toward the victim. Once the person has been pulled out of the hole in the ice, he or she should lie flat on the ice to be pulled to safety.

Ice can be dangerous -- but it can also be beautiful. There are few things as pretty as the sun shining on ice-coated twigs the morning after a storm, for example. This winter, take time to notice the changes that falling temperatures bring to your familiar surroundings.

Before your kids gear up for winter sports, make sure they know simple safety rules, the American Academy of Pediatrics advises. Whether ice skating, skiing or sledding, your child should have safe equipment and know how to use it. Teach your children how to stop and fall down correctly. The best way to fall for each of these sports is sideways.

The most common skating injuries are to the lower extremities, such as the ankles. To minimize problems, select skates that fit well, not ones that your child can "grow into." Oversize skates are unstable and more likely to cause falls. If you're going skating outdoors, call the local park authority to find out if the ice is thick enough and if the rink is supervised. Remind kids to watch for sticks or other objects in the ice that could cause them to trip and fall.

Catherine O'Neill is a children's writer.