Free printable SSAT practice test for Grade 8

Suppose a friend gives you a free ticket to a concert on Saturday night. The ticket cost you nothing, so going seems to cost nothing as well. An economist would disagree. If attending means giving up a shift at a part-time job that would have paid sixty dollars, or an evening you had planned to spend finishing a project, then the concert has a real cost after all. It is simply a cost that does not appear on any receipt.

Economists call this opportunity cost: the value of the best alternative you give up when you make a choice. The idea rests on a plain observation about scarcity. Time, money and attention are limited, so every decision to use them one way is also a decision not to use them another way. The old saying that there is no such thing as a free lunch makes the same point. Even when someone else pays for the meal, the hour you spend eating it is an hour you cannot spend on anything else.

Opportunity cost is easy to state and surprisingly easy to forget. A price tag is visible and specific; the alternative a purchase rules out is usually vague and unspoken. As a result, people tend to weigh what they can see. A shopper choosing between two phones may spend an hour comparing features while never asking what else the extra two hundred dollars could have bought. Researchers who study decision making have found that simply reminding people of what else they could do with their money can make them noticeably more cautious about spending it.

The concept also explains choices that look puzzling at first. Consider the full cost of attending college. Tuition and books are the obvious expenses, but economists add another: the wages a student could have earned by working full time during those years. For most students, the investment still pays off over a lifetime. For a rare few with an unusually valuable opportunity, such as a promising young athlete, the income given up may be large enough that delaying college makes sense.

It is worth separating opportunity cost from a related idea that often causes confusion. A sunk cost is money or effort already spent that cannot be recovered. Because it is gone whichever way you decide, it should not influence what you do next. The person who sits through a dull movie only because the ticket was expensive is letting a sunk cost steer the decision, while ignoring the opportunity cost of the next two hours.

None of this means every choice requires a spreadsheet. The value of the concept lies in a single habit of mind: asking, before committing, "Compared with what?" That question turns invisible costs into visible ones, and visible costs are far easier to weigh.

Far below the reach of sunlight, in water only a few degrees above freezing, it is always snowing. The flakes are not ice. They are clumps of dead and dying matter: the remains of microscopic algae, the shed shells of tiny animals, waste pellets and grains of dust blown off distant deserts, all bound together by a sticky film of mucus that many sea creatures produce. Ocean scientists call this steady drift "marine snow," and although a single flake is hardly worth noticing, the snowfall as a whole is one of the planet's most important means of moving carbon.

The process begins at the sunlit surface. Phytoplankton, single-celled organisms that float near the top of the sea, absorb carbon dioxide and use sunlight to turn it into living tissue, much as land plants do. Phytoplankton multiply quickly and die quickly, and they are eaten by small grazing animals that in turn release compact waste pellets. Each of these fragments contains carbon that was, not long before, dissolved in the water and in contact with the air above it.

Most of that material never travels far. Bacteria break much of it down long before it leaves the upper ocean, releasing the carbon back into the water, where it can return to the atmosphere. But some particles collide, stick together and grow heavy enough to sink faster than they can be consumed. A clump may spend weeks falling, and along the way it serves as a moving meal. Animals of the deep, from small crustaceans to the vampire squid, which gathers drifting particles with long, sticky filaments, depend on this rain from above in a place where almost nothing grows.

Only a small fraction of the original carbon reaches the deep ocean or the seafloor. That fraction matters enormously, however. Carbon carried into the deep can remain isolated from the atmosphere for centuries, and some of it is buried in sediment for far longer. Scientists refer to this whole chain, from surface growth to deep storage, as the "biological pump." Without it, the amount of carbon dioxide in the air would be considerably higher than it is.

Researchers study marine snow with sediment traps, which are containers anchored at various depths to catch what falls, and increasingly with cameras and robotic floats that can count particles as they pass. Their measurements remain uncertain, partly because the ocean is vast and the snowfall varies with season and place. A spring bloom of phytoplankton can send down a heavy burst; a nutrient-poor stretch of open ocean may produce only a thin drizzle.

What the research makes clear is that the deep sea, often imagined as remote and unchanging, is closely tied to the surface world. The carbon in a flake drifting past a vampire squid may have been in the air above the waves only a season earlier.