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The cost of trainers could be about to get a lot more expensive for people living in America.

That's because US President Donald Trump has put extra costs, or tariffs, on footwear and other goods made in China that are then sold in the US.

Now 173 footwear companies, including Nike, Adidas and Converse, have signed a letter asking the president to stop the extra tariffs as they'll make trainers too expensive.

The tariffs have been brought in because of an ongoing trade war between the US and China.

What is a trade war?

Around the world countries buy and sell from each other - that's trade.

By selling their goods in other countries, people and companies make more money.

That can be anything from consumer goods - like handbags or trainers - to industrial goods like steel and railway equipment.

A trade war is when two or more countries decides to restrict each other's ability to sell stuff in their country.

Country A raises their costs (those tariffs we were talking about) and then the other country, Country B, responds by increasing their tariffs on goods brought in from Country A. Still with us?

Those extra costs make imported goods more expensive and consumers sometimes stop buying as many of them, and instead could decide to buy things made in their own country.

So why is there a trade war between the US and China?

Good question. There are a couple of big reasons.

Donald Trump has been clear for a long time that he isn't happy with the way China trades with other countries - since before he became US president in 2016 in fact.

He says they use unfair trade practices. He says some Chinese companies copy US designs and ideas then sell these products back to the US at a lower price.

He says this has a negative impact on the American economy.

Donald Trump is also concerned that the US buys too much from China and doesn't make enough money from selling to them.

Will the trade war affect the prices in the UK?

Financial experts say there's no obvious direct impact on consumers in the UK but the trade war is generally seen as a threat to economies around the world.

Because China and the US are the two largest economies in the world they are both very important places for other countries to sell their goods to.

Problems in their relationship can affect other nations and tension around who can sell what to whom can cause problems for everyone.

Are there any winners in the trade war?

If the flow of goods imported into a country is slowed down, it can lead to a growth in things that are made domestically, so that can help business.

But the problems come if a) companies make the products using raw materials bought abroad or b) also want to sell their product abroad.

How will the trade war end?

Both US and international firms have said they are being harmed by the tariffs imposed by the US and China.

And worries about the trade war ramping up further and the uncertainty that would bring is having an impact on trading around the world.

US President Donald Trump has said the US will make a deal with China "when the time is right."

But the truth is, we don't know when that will happen.