You have heard people say the market was up today. There is no such thing as the market. There are indexes, and an index is just a list of companies with a scoreboard attached. Understanding that one sentence explains most of what happens on financial news.
What Happened
S&P Dow Jones Indices announced that Nike will be removed from the S&P 100 before the market opens on September 21, 2026. Technology companies including Dell, Palo Alto Networks, Arista Networks and SanDisk take its place. Nike is not disappearing from the market. It stays in the much larger S&P 500. But the S&P 100 is the short list of the very biggest American companies, and Nike no longer qualifies after losing roughly 77 percent of its value since 2021.
Why It Matters
An index is not just a scoreboard. Trillions of dollars are invested in funds whose entire job is to copy one. If a fund promises to track the S&P 100, it must hold exactly the companies on that list in roughly the right proportions. When Nike comes off the list, every one of those funds has to sell its Nike shares. Not because a manager decided Nike is a bad company. Because the rules say so. That is called forced selling, and it can push a price down on its own, independent of anything happening inside the business.
The Concept: A List, a Weighting, and a Mirror
Three ideas do most of the work. First, the list. Someone decides which companies are in. For the S&P 500 and S&P 100, that someone is a committee at S&P Dow Jones Indices, using rules about size, profitability and how much of the stock trades freely. Second, the weighting. Most major indexes are market-cap weighted, meaning bigger companies count more. A giant company moving 1 percent shifts the index far more than a small one moving 10 percent. So when you hear the S&P 500 was up today, you are mostly hearing about its largest members. Third, the mirror. An index fund buys the list so you do not have to. Instead of picking winners, you own a sliver of all of them and accept the average. That idea is the single most common way ordinary people invest, and it only works because the index is a transparent, rule-based list.
Indexes also let you see inside the market. In 2026, eight of the eleven S&P sectors are up, with energy leading at about 43 percent, while consumer discretionary is down roughly 2.3 percent. That one line tells you that oil did well and the companies selling optional stuff to shoppers did not.
"An index fund does not try to beat the list. It just agrees to be the list."
Why Teens Should Care
When you eventually open a retirement or brokerage account, the default option will almost certainly be an index fund. Most first investments are, whether people realize it or not. Knowing what that actually means matters more than knowing any single stock ticker. It also gives you a useful defense against headlines. When a story says the market crashed, you now know to ask which index, made of which companies, weighted how. The Dow tracks 30 companies. The S&P 500 tracks 500. The Nasdaq Composite leans heavily technology. Those three can tell three different stories about the same day, and all three can be accurate.
An index is a list with rules. Owning the list, instead of guessing which name wins, is how most people actually invest.
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Sources
S&P Dow Jones Indices · CNBC · Barchart · Yahoo Finance