Robert Krulwich

When a species gets rare, its market value rises. The higher its price, the more it's hunted. The more it's hunted, the rarer it gets. Not a happy cycle, and this keeps happening ...

Add all of us up, all 7 billion human beings on earth, and clumped together we weigh roughly 750 billion pounds. That, says Harvard biologist E.O. Wilson, is more than 100 times the biomass of any large animal that's ever walked the Earth. And we're still multiplying. Most demographers say we will hit 9 billion before we peak, and what happens then?

I guess everybody, even the smartest people who ever lived, have days when they feel dumb — really, really dumb. Oct. 1, 1861, was that kind of day for Charles Darwin.

In a letter to his friend Charles Lyell, Darwin says, "I am very poorly today," and then — and I want you to see this exactly as he wrote it, so you know this isn't a fake; it comes from the library of the American Philosophical Society, courtesy of their librarian Charles Greifenstein. Can you read it?

It says:

Whoah! You know the feeling, right?

Oh, dear.

First off, this lizard? It's not really a lizard. It's an almost vanished species, a reptile like no other.

Here's a question you probably didn't know was a question: Why is the sky dark at night?

Look at this map, and notice that deep, deep in the Republican South, there's a thin blue band stretching from the Carolinas through Georgia, Alabama and Mississippi. These are the counties that went for Obama in the last election. A blue crescent in a sea of red.

You are 200 miles directly above the Martian surface — looking down. This image was taken by the Mars Reconnaissance Orbiter on Jan. 27, 2010. (The color was added later.) What do we see? Well, sand, mostly. As you scroll down, there's a ridge crossing through the image, then a plain, then dunes, but keep looking. You will notice, when you get to the dunes, there are little black flecks dotting the ridges, mostly on the sunny side, like sunbathing spiders sitting in rows. Can you see them?

There was a time — and it wasn't that long ago — when kids would leave home on a summer morning and roam free. "I knew kids who were pushed out the door at eight in the morning," writes Bill Bryson of his childhood in the 1950s, "and not allowed back until five unless they were on fire or actively bleeding." That's what kids did. They went out. Parents let them, and everybody did it. "If you stood on any corner with a bike — any corner anywhere — more than a hundred children, many of whom you had never seen before, would appear and ask you where you were going," Bryson writes.

In yesterday's post, I crowned an Oxford geologist William Buckland as Most Daring Eater Ever. And he was. But I think he deserves one additional, albeit smaller, honor.

Before we go slow, let's go fast, so fast you can't go any faster. That would be light in a vacuum, traveling at 670 million miles per hour ...

Light, of course, can slow down. When light passes through water, it loses speed. A diamond is an even better speed bump. It can slow a beam of light by 40 percent.

But moving on, you and I are going pretty fast right now, though we don't notice. The planet we're on is zipping around the sun at 66,000-plus miles per hour ...

So you're minding your own business when all of a sudden, a nuclear bomb goes off, there's a shock wave, fires all around, general destruction and you, having somehow survived, need a drink. What can you do? There is no running water, not where you are. But there is a convenience store. It's been crushed by the shock wave, but there are still bottles of beer, Coke and diet soda intact on the floor.

So you wonder: Can I grab one of those beers and gulp it down? Or is it too radioactive? And what about taste? If I drink it, will it taste OK?

Here's an old, old, question, but this time with a surprise twist. The question is — and I bet you asked it when you were 8 years old and sitting on a beach: Which are there more of — grains of sand on the Earth or stars in the sky?

Obviously, grains and stars can't be counted, not literally. But you can guestimate.

What we have here is a moon — a small one (slightly wider than the state of Arizona) — circling Saturn.

If you look closely, you will see a small splay of light at its top, looking like a circular fountain.

That's because it is a fountain — of sorts. A bunch of volcano-like jets are sending fantastically high geysers of water vapor up into the sky, so high that you can see them in this remarkable print by Michael Benson, back lit by light bouncing off of Saturn.

Check out this graph of America's "Growing Season" — it measures the number of continuous days and nights when it never gets below 32 degrees. You could call this our "frost-free" time of year. In many places, the frost-free season begins in the spring and ends somewhere in October.

As you can see, over the 20^th century, it's been staying frost-free longer...and longer...and longer...

Yes, it was an amazing landing, an engineering triumph, a 150-million-mile slam dunk, spectacular in every way, except ... I think my grandpa would be disappointed. I'm not sure of this, since he died 50 years ago, but I have a hunch.

It starts with a handwritten letter he wrote back in 1907. He was a travelling salesman. He sold men's hats, and his job was to visit retailers all over the country. "One evening," he wrote, "train riding between Chicago and Kansas City or St. Louis, sitting the club car, I read a magazine, The Century..."

This is one of those fun-to-think-about questions. A brain isn't much to look at, after all. It's about the size of your two fists put together, three pounds to hold, but oh my, what it can do.

With our brains, we can think backwards, imagine forwards, conjure, create things that don't exist, leap vast distances. For example, suppose I say to you, close your eyes and imagine this:

It's hot today. Really, really, hot; over a 100 degrees Fahrenheit hot, and so I'm sweating.