Listen to an Academic Talk

Narrator: Listen to part of a talk in a biology class.

Woman: Today I want to talk about a kind of camouflage that's so common we hardly notice it. It's called countershading. Look at a shark, a deer, or many birds, and you'll see that the animal's back is darker than its belly. So why would that help? Well, think about how light works outdoors. Sunlight comes from above, so the top of an object is brightly lit and the bottom is in shadow. That shadow is actually one of the main ways our eyes judge an object's shape. Now, if an animal's belly is lighter, it cancels out that shadow, so the animal looks flatter and is harder to pick out from the background. For animals that live in the sea, there's a second benefit. A predator looking down sees a dark back against the dark water below, while a predator looking up sees a pale belly against the bright surface. So countershading helps an animal blend in from more than one direction.

Narrator: Listen to part of a talk in an astronomy class.

Woman: You've probably noticed that the Moon always shows us the same face. The craters and dark patches you can see tonight are the same ones people saw a thousand years ago. A lot of people conclude from this that the Moon doesn't rotate at all. Actually, it does. It turns once on its axis in about twenty-seven days, which is exactly the time it takes to orbit Earth once. Because those two periods match, the same side always faces us. Astronomers call this tidal locking. Here's how it happens. Earth's gravity pulls slightly harder on the near side of the Moon than on the far side, so the Moon is stretched a little. Billions of years ago, when the Moon was spinning faster, that stretched shape was constantly being pulled out of line, and the friction gradually slowed the spin until it matched the orbit. And this isn't unusual. Most of the large moons in our solar system are tidally locked to their planets.

Narrator: Listen to part of a talk in an economics class.

Woman: When economists talk about cost, they don't just mean money. Let's take an example. Say you have a free ticket to a concert on Saturday night. Free, right? Well, not exactly. Suppose that on the same night, you could have worked a shift at your part-time job and earned eighty dollars. By going to the concert, you give up that eighty dollars. Economists call this the opportunity cost: the value of the best alternative you give up when you make a choice. Notice I said the best alternative, not all of them. Maybe you could also have studied or seen a movie, but you only count the single most valuable option you gave up. This idea explains a lot of behavior that seems strange at first. For example, people who earn very high hourly wages often pay someone else to clean their homes, even if they enjoy cleaning. Their time is simply too valuable to spend that way.