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Question: Why can you sometimes see, just barely, the dark part of a crescent moon?

Answer: The moon has an incredible number of tricks up its sleeve: Solar eclipses, lunar eclipses, full moons, half moons, crescents, the optical illusion of looking huge when it's low on the horizon, the peculiarity of having one side permanently facing away from the Earth, and so on. The moon is an odd ball. Nothing else in the sky has such conspicuous phases. You'd never see a crescent sun, except in an eclipse.

The next time you look at a crescent moon, look closer. You may be able to see the rest of the moon - the darkened part. That part of the moon is turned away from the sun. But if it's night on that part of the moon, why isn't the surface too dark to see?

The Greeks wondered about this in ancient times, and guessed that the moon was slightly transparent, that some light was leaking into the dark part from the far side. Another theory arose in the 16th century that the moon's surface was exhibiting phosphorescence. (Centuries later we learned that the moon does, indeed, phosphoresce, but that's not why we see the dark part.)

To find the right explanation, you just have to imagine yourself on the moon's surface, say, somewhere on the Sea of Tranquillity, where Apollo 11 landed. What would you see in the night sky?

Right: Earth.

A full Earth, almost. A big, bright, gibbous Earth. And the Earth is casting light on the moon. We can see the dark part of the moon because of Earthshine.

The sun shines on the Earth, the light is reflected to the moon, and then reflected back to Earth. You lose a lot of brilliance in all that bouncing around but there's still enough to make the dark part of the moon visible.

Richard Berendzen, professor of physics at American University, says that Earth is actually shinier than the moon. The moon's albedo is 7 percent - meaning that 93 percent of the light that hits it is absorbed, and 7 percent is reflected back into space. The Earth's albedo is 39 percent. Venus is brightest of all, with an albedo of 65 percent.

Clouds are a big factor. They're white. Venus is covered with clouds, which is why it's so reflective. Earthshine is greatest when Earth is all clouded up.

We're told that Earthshine makes it possible to sometimes see a "new" moon, the moon's most shadowy phase.

Question: Why do we have two sets of teeth in our lifetime, and not one or three or 10 or whatever?

Answer: The thing about teeth is: You really get to know them. You know your teeth better than you know certain members of your immediate family. You know the grooves, the gaps, the misalignments, the protrusions. You even develop a sense of how different teeth have different personalities - there's the jauntiness of the incisors, the workmanlike sobriety of the first molars, the thinly disguised barbarism of the canines.

The question is, why don't we have just one set for an entire lifetime? What's with this "milk teeth" nonsense?

And if the answer is that it's a wonderful thing to replace teeth with a new set, then why don't we keep replacing them, over and over, so that we don't have to worry about root canals and dentures?

The main thing you should remember is that humans are diphyodonts. That means we have two sets of teeth. Elephants, by contrast, have five sets in their lifetime. (We do not know the fancy word for that - pentadonts, maybe?)

In humans both sets of teeth are present in the mouths of newborns, one on top of the other, with the milk (also known as "primary," or "deciduous") teeth being more developed than the seed-like permanent (secondary) teeth underneath. There's an advantage in having this two-set system. The mouth of a child is just too small to accommodate a bunch of big, permanent teeth, so we start with the 20 milk teeth.

These teeth, in addition to making it possible to eat, influence the formation of "arch space" for the later permanent teeth. The milk teeth aren't just for chewing food, they're staking out turf on behalf of the big chompers down below. If a child accidentally loses his or her milk teeth, especially the cuspids and molars, it can mess up the arches and result in malpositioning of the permanent teeth.

And finally, the milk teeth are crucial for the articulation of speech.

Why don't our "secondary" teeth fall out at, say, the age of 40, and get replaced by "tertiary" teeth? Partly because humans during most of their evolutionary history didn't live that long anyway. But another reason is that until the invention of refined sugar, humans didn't have a big problem with cavities.

Beavers, by the way, have only one set of teeth per lifetime. So why don't their teeth get worn down to nubs by all that gnawing on wood? Because their teeth keep growing. A beaver needs to keep busy building dams and whatnot or else it will literally get too long in the tooth.