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When winter weather hits and and snow abound, all sorts of surfaces can get and . If you're a driver, this is not such a good thing. and snow can make driving treacherous .

Of course, if you want to go sledding, it's a great thing! When it comes to sliding down a hill on a or a , you want the snow to pack down into that's as as possible.

No one would deny that is , but have you ever thought to wonder why? After all, is a solid, right? Can you think of any other solids that are ?

Think of a player for a moment. Can you imagine trying to on a sidewalk? How about a wooden floor? These are solids, too, so why aren't they ?

After years and years of research, scientists can explain what makes ...but not exactly why. So what makes ? The answer is definitely water - the liquid form, not the solid!

Scientists have discovered that a very thin layer of liquid water exists at the edge of - even that is extremely cold. For years, scientists believed that this thin layer of liquid water was the result of being applied to .

Water has an unusual property: its solid form () is less dense than its liquid form. This is why cubes float in a glass of water. The lower of means that its melting temperature can be lowered by applying .

So, for example, when an glides over , scientists once believed that the applied by the would lower the melting point of the solid . This would cause the layer to melt. The would then slide across on a thin sheet of water that would immediately refreeze as soon as the passed.

Unfortunately, scientists later discovered that the applied by skates or people's shoes standing on was not great enough to cause these changes. So, they had to look for another explanation. In recent years, two other theories have been developed to explain the presence of the thin liquid water layer that exists on .

One of these theories is that causes the liquid layer of water to form on . is the force that generates heat whenever two objects slide against each other. If you rub your hands together, you can feel them heat up. That's at work. When a moves over the surface of , the between the and the generates heat that melts the layer of .

But is still even when you're standing still. If you stand on without moving, no exists to generate heat, yet the is still . So there must be something else going on.

The other theory is that is just , because the layer never turns to a solid. According to this theory, the water molecules at the surface of the move more, because they're at the edge and there aren't any molecules above them to help keep them in place. As a result, the layer stays in a liquid state even at temperatures way below freezing.

So far, scientists haven't been able to decide which of these two theories is correct. Without a definitive answer at this point, it's safe to say that both theories play a role in keeping .

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When winter weather hits and and snow abound, all sorts of surfaces can get and . If you're a driver, this is not such a good thing. and snow can make driving treacherous . Of course, if you want to go sledding, it's a great thing!

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Do you ever gaze out the window while your parents are driving you to and from school, church, or soccer practice? As the signs fly by, do you ever WONDER what all those numbers and interesting symbols mean?

There's one , in particular, that you may have seen from time to time. It usually looks like a with a couple lines underneath it. What could this funny-looking mean? Caution: earthworms ahead? Don't drive over spaghetti? Bacon will make your fly?

While all those are funny possibilities, that curious actually means when . It's a reminder to drivers to be extra- when driving during or shortly after it rains.

If you've ever played in the street during or after a rainstorm, you may be about such a . After all, a street probably doesn't seem all that when you're in your tennis shoes. Why would it be dangerous to drive on?

Before examining the effects of on a , let's first take a look at what force helps to keep cars on the street and drivers in control. When rubber tires meet the , their motion across the creates a force known as .

For example, the weight of a pushing down on its tires pushes the rubber of the tire into the thousands of tiny pits that make up a typical . Vibrations from the combine with the deformation of the rubber to create heat, producing that helps the tires "stick" to the .

When it rains, the on a causes a loss of . As tires move over a , the fills in the tiny pits in the , effectively smoothing out the . As a result, the normal heat and created is decreased, leading to a that is more than when dry.

If there's a lot of on the , including standing in puddles, even more severe loss of can occur. In these cases, a 's tires can completely lose contact with the as they surf along on a thin layer of . When this occurs, it's called hydroplaning , and it can be very dangerous.

There are also substances on roads that can lead to a loss of when is added. For example, most dry roads contain a layer of tar, rubber, and oil. When it starts to , these substances can mix with the , creating a greasy layer that can be very .

So what can you do to prevent accidents when you're driving in conditions? The first rule of driving on potentially- surfaces is slow down! Feel free to drive well below the speed limit if necessary to help maintain control of your vehicle. If it is raining severely and hydroplaning is a possibility, pull over to the side of the until conditions improve. Lastly, make sure your tires are in good condition and properly .

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Do you ever gaze out the window while your parents are driving you to and from school, church, or soccer practice? As the signs fly by, do you ever WONDER what all those numbers and interesting symbols mean? There's one , in particular, that you may have seen from time to time.

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What do you patterns do you notice?

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Learn about conservation of energy with a skater gal! Explore different tracks and view the kinetic energy, potential energy and friction as she moves. Build your own tracks, ramps, and jumps for the skater.

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Post a video, drawing, or comment explaining what you've learned about forces (and friction) after looking at these sources.

These resources show that forces are ____.  For example ________.  I also learned _____.