How does the exchange rate of a currency change every day? Why does that happen?
Please explain to me with a naive approach and avoiding financial jargon.
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It's simply supply and demand.
If you're an importer trying to buy from overseas, you'll need foreign currency, maybe Euros. Or if you want to make a trip to Europe you'll need to buy Euros. Or if you're a speculator and think the USD will fall in value, you'll probably buy Euros.
Unless there's someone willing to sell you Euros for dollars, you can't get any. There are millions of people trying to exchange currency all over the world. If more want to buy USD, than that demand will positively influence the price of the USD (as measured in Euros). If more people want to buy Euros, well, vice versa.
There are so many of these transactions globally, and the number of people and the nature of these transactions change so continuously, that the prices (exchange rates) for these currencies fluctuate continuously and smoothly.
Demand is also impacted by what people want to buy and how much they want to buy it. If people generally want to invest their savings in stocks instead of dollars, i.e., if lots of people are attempting to buy stocks (by exchanging their dollars for stock), then the demand for the dollar is lower and the demand for stocks is higher. When the stock market crashes, you'll often see a spike in the exchange rate for the dollar, because people are trying to exchange stocks for dollars (this represents a lot of demand for dollars).
Then there's "Supply:"
It may seem like there are a fixed number of bills out there, or that supply only changes when Bernanke prints money, but there's actually a lot more to it than that. If you're coming from Europe and want to buy some USD from the bank, well, how much USD does the bank "have" and what does it mean for them to have money? The bank gets money from depositors, or from lenders. If one person puts money in a deposit account, and then the bank borrows that money from the account and lends it to a home buyer in the form of a mortgage, the same dollar is being used by two people. The home buyer might use that money to hire a carpenter, and the carpenter might put the dollar back into a bank account, and the same dollar might get lent out again. In economics this is called the "multiplier effect."
The full supply of money being used ends up becoming harder to calculate with this kind of debt and re-lending.
Since money is something used and needed for conducting of transactions, the number of transactions being conducted (sometimes on credit) affects the "supply" of money.
Demand and supply blur a bit when you consider people who hoard cash. If I fear the stock market, I might keep all my money in dollars. This takes cash away from companies who could invest it, takes the cash out of the pool of money being used for transactions, and leaves it waiting under my mattress. You could think of my hoarding as a type of demand for currency, or you could think of it as a reduction in the supply of currency available to conduct transactions.
The full picture can be a bit more complicated, if you look at every way currencies are used globally, with swaps and various exchange contracts and futures, but this gives the basic story of where prices come from, that they are not set by some price fixer but are driven by market forces.
The bank just facilitates transactions. If the last price (exchange rate) is 1.2 Dollars per Euro, and the bank gets more requests to buy USD for Euros than Euros for USD, it adjusts the rate downwards until the buying pressure is even. If the USD gets more expensive, at some point fewer people will want to buy it (or want to buy products from the US that cost USD). The bank maintains a spread (like buy for 1.19 and sell for 1.21) so it can take a profit.
You should think of currency like any other commodity, and consider purchases for currency as a form of barter. The value of currency is merely a convention, but it works. The currency is needed in transactions, so it maintains value in this global market of bartering goods/services and other currencies. As supply and demand for this and other commodities/goods/services fluctuate, so does the quantity of any particular currency necessary to conduct any of these transactions.
A official "basket of goods" and the price of those goods is used to determine consumer price indexes / inflation etc. The official price of this particular basket of goods is not a fundamental driver of exchange rates on a day to day basis.
The basic idea is that money's worth is dependent on what it can be used to buy. The principal driver of monetary exchange (using one type of currency to "buy" another) is that usually, transactions for goods or services in a particular country must be made using that country's official currency. So, if the U.S. has something very valuable (let's say iPhones) that people in other countries want to buy, they have to buy dollars and then use those dollars to buy the consumer electronics from sellers in the U.S.
Each country has a "basket" of things they produce that another country will want, and a "shopping list" of things of value they want from that other country. The net difference in value between the basket and shopping list determines the relative demand for one currency over another; the dollar might gain value relative to the Euro (and thus a Euro will buy fewer dollars) because Europeans want iPhones more than Americans want BMWs, or conversely the Euro can gain strength against the dollar because Americans want BMWs more than Europeans want iPhones. The fact that iPhones are actually made in China kind of plays into it, kind of not; Apple pays the Chinese in Yuan to make them, then receives dollars from international buyers and ships the iPhones to them, making both the Yuan and the dollar more valuable than the Euro or other currencies.
The total amount of a currency in circulation can also affect relative prices. Right now the American Fed is pumping billions of dollars a day into the U.S. economy. This means there's a lot of dollars floating around, so they're easy to get and thus demand for them decreases.
It's more complex than that (for instance, the dollar is also used as the international standard for trade in oil; you want oil, you pay for it in dollars, increasing demand for dollars even when the United States doesn't actually put any oil on the market to sell), but basically think of different currencies as having value in and of themselves, and that value is affected by how much the market wants that currency.
Money is money because people believe it is money. By "believe it is money", I mean that they expect they will be able to turn it into useful goods or services (food, rent, houses, truckloads full of iron ore, mining equipment, massages at the spa, helicopter rides, iPads, greenhouses, income streams to support your future retirement, etc).
Foreign exchange rates change because people's ideas about how much useful goods or services they can get with various currencies change. For example: if the Zimbabwe government suddenly printed 10 times as much money as used to exist, you probably couldn't use that money to buy as much food at the Zimbabwe-Mart, so you wouldn't be willing to give people as many US-dollars (which can buy food at the US-Mart) for a Zimbabwe-dollar as you used to be able to.
(It's not exactly that easy, because - for instance - food in the US is more useful to me than food in Zimbabwe. But people still move around all sorts of things, like oil, or agricultural products, or minerals, or electronics components.)
The two main things that affect the value of a currency are the size of the economy that it's tied to (how much stuff there is to get), and how much of the currency there is / how fast it's moving around the economy (which tells you how much money there is to get it with). So most exchange rate shifts reflect a change in people's expectations for a regional economy, or the size of a money supply.
(Also, Zimbabwe is doing much better now that it's ditched their own currency - they kept printing trillions of dollars' worth - and just trade in US dollars. Their economy still needs some work, but... better.)