Scarcity and surplus seem like opposite concepts, and they are often confused. But surplus is not the opposite of scarcity, and a surplus good is not the opposite of a scarce good. There’s no paradox here: take Christmas trees, for example.

Fresh-cut Christmas trees, an icon of the holiday season, are a highly desirable consumer item, at least through Christmas Eve. They are a scarce good in the economic sense. That means that someone had to devote resources to the process of growing, shaping, finding, cutting, or otherwise acquiring the trees. Consumers find that fresh-cut trees are widely available for sale during the weeks leading up to Christmas. Though scarce, there is usually no shortage of Christmas trees in the market. Instead, there is often a surplus.

The opposite of a scarce good is a “found” or “free” good. Goods stop being free as soon as someone devotes resources, including effort, into locating, producing, using, or gathering them. Free goods are almost nonexistent. Even air is almost never free, once we consider that we nearly always heat or cool it, dehumidify it, or blow it around to make ourselves more comfortable. Since we have to devote resources to making air more comfortable throughout most of the year, air—at least heated or air-conditioned air—is both a good (desirable) and it is scarce (not just sitting there at the right temperature and humidity).

Scarce goods needn’t always have a money price, though they usually do. On Robinson Crusoe’s island, clean water is a scarce good with a zero money price. Robinson would probably prefer to pay someone else to capture, filter, bottle, and deliver water to his door, but he doesn’t have the option. So a zero money price doesn’t guarantee that the good requires no resources to obtain. But as Robinson’s situation illustrates, a scarce good will sometimes have a zero money price.

What about “found” items—the ones that really can be gotten through absolutely no effort or resource expenditure at all? Free or “found” items, most lawn weeds, for example, aren’t goods in an economic sense. We don’t value or want more of them. I don’t take any special pains to sow dandelion seeds in my lawn, because for me they are a “bad.” I want less of them. I can even be induced to buy weed killer, a “good,” to get rid of the dandelions. The upshot: economic bads are not scarce because we don’t want more of them. Only goods—desirable items—can be scarce.

And now back to the Christmas trees. Tree sellers know that consumer interest fades dramatically once the Christmas holiday arrives, and every year we see Christmas tree lots and stores stocked with some trees that did not sell. If the trees are scarce, and they are, why is there a surplus?

The answer is price. The unsold trees—the surplus—could have been sold and cleared from the market if vendors were willing to drop tree prices as consumer interest declined. Granted, this could mean waiting until almost the last moment to discount the remaining stock. But as Charlie Brown once demonstrated, every tree has a market-clearing price (OK, his was an artificial model).

Sale prices are used to liquidate stocks of merchandise economy wide. Retailers use price cuts to clear out winter coats before the spring season, to sell remaining summer merchandise before back-to-school promotions, and to unload Christmas cards, ornaments, and decorations before Valentine’s Day chocolates arrive to fill store shelves. All of these out-of-season items are also scarce goods.

Timing a sale is obviously more critical for perishable goods than for non-perishables, but the market-clearing concept is the same. You could conceivably store your sweetheart’s sale chocolates until next Valentine’s Day, for example, but that probably won’t work with a Christmas-bound fresh-cut Frazier fir.

Without price adjustments, at least one side of the fresh Christmas tree market will be disappointed and left with unsold trees. With the right price adjustments, markets that initially misjudged supply and demand can still wind up coordinating everyone’s plans.

As for your sweetheart, if you offer year-old chocolates, or a February 15th discount version, you may not have any significant plans to worry about anyway.