Some authors state that bodies of water are blue because the water reflects the sky. But wouldn't this only make the sky-reflections blue? And doesn't water remain blue on cloudy days? Exactly. There's no mystery here; water looks blue because water *is* blue. It's not the sky that creates the color.

But what if you pour yourself a drink; in that case the water is clear, right? Well, it's not blue as far as your eyes can tell. But if the water in your cup was very very slightly blue, you'd never see it. You'd only notice the blue color if your cup was many feet deep.

In fact, this is exactly how it works: water is clear, but it's very very slightly blue. A small amount of water is too thin, so a small amount looks clear rather than blue. But look through thirty feet of water, and you'll see a strong color. Gaze into a hundred feet of deep pure mountain lake water, and you'll see exactly what color the water actually has. Yet if you scoop a canteen full of that lake water, it will look totally clear.

This one isn't purely an error. Still, it involves misconceptions on the part of authors.

Why is the sky blue? Usually the books start going on about wavelengths of light, Tyndall effect, and Rayleigh scattering. They teach some complicated physics first, then use it to explain blue sky and sunsets. Their explanations are correct. But what happens when you don't understand the physics? Doesn't this make their explanation useless? And do you just give up?

They're wrong: you don't need complicated physics to understand this. The sky is blue for a very simple reason:

It's true that small amounts of air are almost perfectly transparent. So are small amounts of water. Go to an opaque muddy river or pond and use a glass to dip out a cup of water. The water looks clear, no? Yet the river is opaque brown. When you try to look through ten cups of water, or a hundred cups, the water seems to turn into brown paint. Yet a single cup of river water almost looks clean.

Air behaves like this too. A mile of air looks clear, but ten miles of air looks misty blue, and a thousand miles of air looks opaque white. The air is acting like the dirty river water, where a thin layer looks clear but a thick layer doesn't.

The sky is blue because air is a powdery blue material, and when the sun shines on it, you can see this blue color. Each molecule of air behaves like a bluish mote of dust. Stare upwards on a sunny day, and you're looking into a thick cloud of air. (There really is no "sky" up there. You're not looking at a blue surface. Instead you're just seeing the Earth's layer of blue air against the blackness of outer space.)

Suppose you could go far out into space away from the Earth, then build yourself a thin hollow glass bubble a thousand miles wide. Viewed from the Earth, your empty glass bubble would be almost invisible. OK, now fill your bubble with air. It won't be invisible any more. It will look like a giant droplet of bright blue paint. It might even look whitish in the middle, since very thick layers of air seem as white as milk. What if you let your giant glass bubble crash into the moon? The air inside would pour out over the moon's surface and form a thick temporary layer of atmosphere. The moon wouldn't look white anymore. It would turn blue.

OK, now here's a question. Smoke is white, milk is white, and powder is white. A big cloud of particles should look like white smoke, not like a blue dye. Why is air blue instead of white? And even more important, why are sunsets red? (Does this mean that air is also a red substance?!!)

Ah, if you start wondering about such things, then *now* you finally need the advanced physics explanations. Nearly all books will explain why an air molecule looks like a bluish dust mote.

Many books contain an incorrect experiment which purports to directly demonstrate that air has weight. A crude beam-balance is constructed using a meter stick. Deflated rubber balloons are attached to the ends, and the balance is adjusted. One balloon is then inflated, and that end of the balance-beam is supposed to sag downwards. A large amount of air supposedly weighs more than a small amount of air.

Unfortunately this experiment lies. When immersed in atmosphere, buoyancy causes full and empty balloons to weigh the same. But then why does the above experiment work? It doesn't! The experiment will fail unless you know the trick: blow the balloon up near to bursting. It secretly relies on the fact that the air within a high-pressure balloon is denser than air within a low pressure balloon. Obviously this does not DIRECTLY demonstrate anything about the weight of air, and it's dishonest to tell students that it does.

To illustrate the problem, try this instead: attach two opened paper bags to the balance, adjust it, then crush one bag so it contains little air. The balance WILL NOT MOVE. What does this teach your class; that air is... weightless? Yet air does have significant weight. We just can't detect this weight directly by using balloons or paper bags.

Here's a way to make the experiment more honest. Perform the balance-beam experiment again, but blow one balloon REALLY full so the rubber feels hard and the balloon is about to pop. Blow up the second balloon so it is ALMOST full, but still a bit stretchy. Try to keep the balloons the same size. Now the balance will show that, even though the balloons are nearly the same size, the "hard" balloon is heavier. Does this teach misleading things to your class? No, instead it exposes the dishonesty of the original demonstration. In truth, balloons full of air do not weigh more than empty ones. However, COMPRESSED air does weigh more than UNCOMPRESSED air.

What if we lived underwater, how could we use the balance-beam to measure the weight of water directly? The answer is that we cannot. If a water-filled balloon and an uninflated balloon were compared underwater, the experiment would show that they weigh the same, which seems to prove that water is weightless. When underwater, a bag full of water weighs just the same as a flattened bag which contains nothing. The situation with air is identical: if we live our lives immersed within a sea of air, we cannot use a balance to easily detect the actual weight of the air. (In fact, a bathtub full of air weighs about a half kilogram, but we cannot sense this weight while living in an atmosphere.)

It's hard to teach the weight of water to the fishes, and hard to teach the weight of air to human grade-schoolers. These experiments could only work if performed in a vacuum environment (say, on the moon's surface.) We humans are like fish underwater: we're not aware that our ocean of air has any weight.

To better demonstrate the weight of air directly, hook a heavy bottle to a vacuum pump, pump all the air out, seal it, then weigh the bottle. Break the seal and let the air in, then weigh it again. The difference in weight is the weight of the air contained in the bottle. Another: use a balance to compare the weight of two vacuum-containing bottles, then open one of them so it becomes filled with air. The bottles will then weigh differently, and the difference is the true weight of the air in one bottle. Or another: build a balance using upside-down paper bags, then place a candle below one of them, then remove the candle again. That bag rises, indicating that a volume of warm air weighs slightly less than a volume of cool air. (Don't set the bag on fire!!) But note that this candle experiment says nothing simple and direct about the actual weight of a volume of unheated air.

What is the difference between a liquid and a gas? Both are "fluids", both can flow. Gases are USUALLY less dense than liquids, although gases under fiercely high pressure can approach the density of liquids, so that's not a good criterion. The main difference is that gases are a different phase of matter: a gas can be made to condense into a liquid form, and a liquid can be made to evaporate into gas. Another major characteristic: because there are bonds between its particles, when a liquid IS PLACED INTO A VACUUM ENVIRONMENT, it will not expand continuously, while a gas in a vacuum chamber will expand continuously until it hits the walls.

This is very different than the oft-quoted rule that "gases always expand to fill their containers." This rule only works correctly if the container is totally empty: the container must "contain" a good vacuum beforehand. However, we all live in a gas-filled environment. All our containers are pre-filled with air. In our environment, any new quantity of gas will not expand, it will just sit there. If you squirt some carbon dioxide out of a CO₂ fire extinguisher, it will not instantly expand to fill the room. Instead it will pour downwards like an invisible fluid and form a pool on the floor. It behaves similarly to dense sugar-water which was injected into a tank of water: it pours downwards, and only after a very long time it will mix with the rest of the water. "Mixing" is very different than "expanding to fill!" The rule about gases does not involve mixing, instead it involves compressibility and instant expansion into a vacuum.

In an air-filled room, dense gases act much like liquids; they can be poured into a cup or bowl, poured out out onto a tabletop, and then they run off the edge onto the floor where they form an invisible mess. :) Less dense gases will stay where they are put, like smoke or like food coloring which has just been injected into a fishtank. Gas of even lesser density rises and forms a pool on the ceiling. Only in the world of the physicist, where "empty container" always implies a vacuum, does the rule about gasses work properly.

Newton originally published his laws of motion in Latin, and in the English translation, the word "action" was used in a different way than it's usually used today. It was not used to suggest motion. Instead it was used to mean "an acting upon." It was used in much the same way that the word "force" is used today. What Newton's third law of motion means is this:

For every "acting upon", there must be an equal "acting upon" in the opposite direction.

For every FORCE applied, there must be an equal FORCE in the opposite direction.

They are slowed because it takes energy to stir the air. While direct friction between the air and the car's surface does play a part, the work done in stirring the air far exceeds the work done in direct frictional heating. If vehicles did not send air swirls and vortices spinning off as they moved, they would barely be slowed by the air at all. Eventually the swirling air is slowed by friction and ends up warmer, but this occurs long after the vehicle has passed.

Opposite poles attract. If we hold two bar magnets near each other, the "N" pole of one magnet is attracted by the "S" pole of another. If we suspend a bar magnet by a thread, the "N" pole of that magnet will point... toward's Earth's north! Something is wrong here. Shouldn't the "N" pole of a magnet point towards the "S" of the Earth? Alike poles should not attract. Either the "N" and "S" printed on all bar magnets is reversed, or the "N" and "S" on the Earth is backwards. Which is it?

Physics defines "N-type" magnetic poles as being the north-pointing ends of compasses and magnets. Wind an electromagnet coil, see which end points towards the north, and that end is the N pole of the electromagnet. Therefore, the magnetic pole inside the northern hemisphere of the Earth is a south-type magnetic pole. The Earth's northern magnetic pole is an S! It has to be this way, otherwise it would not attract the N-pole of a compass.

This is a long-standing but arbitrary physical standard, much the same as defining electrons as being negative. Like it or not, we are stuck with negative electrons, and seconds which last about 1/100,000 of a day, with backwards Earth poles, with centimeters which are about as wide as a small finger, etc.

Salt is not made of NaCl molecules. Salt is made of a three-dimensional checkerboard of oppositely charged atoms of sodium and chlorine. A salt crystal is like a single gigantic molecule of ClNaClNaClNaClNaClNaClNa. When salt dissolves, it turns into independent atoms. Salt water is not full of "sodium chloride." Instead it is full of sodium and chlorine! The atoms are not poisonous and reactive like sodium metal and chlorine gas because they are electrically charged atoms called "ions." The sodium atoms are missing their outer electron. Because of this, the remaining electrons behave as a filled electron shell, so they cannot easily react and form chemical bonds with other atoms except by electrical attraction. The chlorine has one extra electron and its outer electron shell is complete, so like sodium it too cannot bond with other atoms. These oppositely charged atoms can attract each other and form a salt crystal, but when that crystal dissolves in water, the electrified atoms are pulled away from each other as the water molecules surround them, and they float through the water separately.