David Kestenbaum

College textbooks are expensive. You probably already know this. A new biology or economics book can cost $300.

And prices have been soaring, doubling over the past decade, growing faster than the price of housing, cars, even health care.

But, surprisingly, the amount students actually spend on textbooks has not been rising. In fact, the best data we could find on this shows students have been spending a bit less over time.

The 1964 World's Fair showcased jet packs and new miracles of science. There was an entire house made of Formica. You could wipe it clean with a sponge!

The people who put the fair together tried to imagine how the future would look. Here are a few predictions, and how they actually turned out.

1. We had picture phones back then?

The other day I went down to the little shop in the lobby of our building for a snack. I couldn't decide whether I wanted regular M&M's or Peanut Butter M&M's so I bought them both. On the way back upstairs to the office, I noticed something strange on the labels. Each had cost $1, but the pack of Peanut Butter M&M's was a very tiny bit lighter: 0.06 ounces lighter!

I wanted to know why, so I called a couple of experts and asked for their theories:

Theory No. 1: Peanut Butter M&M's are more expensive to make.

Buying insurance doesn't always feel like it makes economic sense, especially for young healthy people. So why are they still willing to pay?

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A couple years ago, Adrienne McAdory read that the organization that sets the rules for Web addresses was going to allow people to apply for new top-level domains. No longer would there be just the old top-level domains like .com and .gov. People could apply for any new top-level domain they wanted.

McAdory liked the idea, but it took her awhile to figure out the right domain.

"I was thinking about lifestyle," says McAdory. "My girlfriends and I just had incessant conversations about 'Oh, we're not married, can't find the right guy, blah, blah, blah.'"

President Obama has called for increasing the minimum wage, saying it will help some of the poorest Americans. Opponents argue that a higher minimum wage will lead employers to cut jobs.

Figuring out the effect of raising the minimum wage is tough. Ideally you'd like to compare one universe where the minimum was raised against an alternate universe where it remained fixed.

In 1895, legislators in New York state decided to improve working conditions in what at the time could be a deadly profession: baking bread.

"Bakeries are actually extremely dangerous places to work," says Eric Rauchway, a historian at the University of California, Davis. "Because flour is such a fine particulate, if it gets to hang in the air it can catch fire and the whole room can go up in a sheet of flame."

This famous bet — between a biologist and an economist — was over population growth. It started three decades ago, but it helped set the tone for environmental debates that are still happening today.

The biologist at the heart of this bet was Paul Ehrlich at Stanford. He wrote a best-selling book in 1968 called The Population Bomb. It was so popular he appeared on The Tonight Show with Johnny Carson.

We recently published a story about how used clothes that get donated in the U.S. often wind up for sale in markets in Africa. As part of the story, we published some photos of used T-shirts we found in a couple of markets in Kenya.

One shirt in particular caught our eye:

For more on what Bitcoin is and how it works, see our story "What Is Bitcoin?"

Gavin Andresen is chief scientist at the Bitcoin Foundation. I first talked with him about Bitcoin, the virtual currency, back in 2011. I checked back in with him this week, because so much has been going on with Bitcoin lately.

Robert Shiller was surprised when he got the call telling him he'd won the Nobel Memorial Prize in economics — surprised that he'd won (of course), but also surprised that he was sharing the award with Eugene Fama.

"He and I seem to have very different views," Shiller told me. "It's like we're different religions."

In particular, they have very different views about economic bubbles.

"The word 'bubble' drives me nuts, frankly," Fama told me.

For more of our reporting on this story, please see our work in The New York Times Magazine and on This American Life.

Earlier this week, I bought a Treasury bill.

Everybody calls Treasury bills T-bills, and they work like this: The government promises to pay holders of T-bills a specific amount on a specific day in the near future. For the T-bill I bought, the government promised to pay $1,000 on Oct. 31.

I bought the T-bill on Tuesday, before Congress had made the debt-ceiling deal, so it was unclear whether I would get paid back on time.

What would happen if Congress doesn't raise the debt ceiling and the U.S. defaults on its debt later this month? The broad economic implications are unpredictable, but a default could cause huge trouble for the global economy.

But whatever happens to the global economy, one thing is clear: People all over the world who have loaned the U.S. government money won't get paid on time.