Brexit: What's been happening?
On 23 June 2016, the UK made a historic decision. It voted to leave the European Union (EU).
More than 33 million adults voted. Around 52% of them chose to leave the group of 28 countries, while 48% wanted to stay in it.
The UK has been a member of the EU for more than 40 years, so this was an extremely important moment.
Even though the vote happened more than 6 months ago, the UK is currently still a member of the EU.
Leaving the EU is a very significant change, which is why it hasn't happened overnight.
So what has happened in the months since the vote?
What happened after the referendum?
Once the result of the vote was announced, the prime minister at the time David Cameron - who wanted the UK to stay in the EU - said he was going to resign.
So the UK needed a new prime minister before anything else could happen.
Theresa May took over the job the following month in July 2016.
Since then, the UK government has been talking about what sort of relationship it would like with the EU when it leaves.
This is the first time that a member of the EU has left since it was formed, so leaders say they are taking their time to make sure that the process runs smoothly.
What is Article 50?
The UK officially announced to the rest of the EU that it is leaving on 29 March 2017.
Article 50 explains how a country should go about leaving the EU if it no longer wants to be a member.
No country has ever left the group though, so nobody has ever put Article 50 into practice before - until now.
What happens next?
Triggering Article 50 has now started a two-year process, during which UK and EU leaders will discuss what their relationship would be like in future.
From now on, the UK will be left out from making big EU decisions.
However, it would still have to follow EU rules and agreements until it is officially no longer a member.
A two-year period of time would start during which the UK and the EU would need to come to an agreement about their future relationship.
There are many things that need to be discussed and agreed upon before the UK could officially leave the EU, including the following:
The UK is currently part of something called the "single market" - an agreement designed to make it easier for countries in the EU to buy and sell (or trade) things with each other.
The Prime Minister Theresa May says that won't be the case when the UK leaves the EU.
So the UK government and EU leaders will need to work out how they do business together, once the UK is no longer a member of the group.
The US, for example, currently sells products into this single market, but it has slightly different rules - for example, it may have to pay extra charges.
So the EU needs to agree how the UK will buy and sell things with them too.
Around three million people from other EU countries currently live, work or study in the UK, and around two million British people live in other EU countries.
Leaders will need to decide whether these people will be able to stay where they are just as before, or if that will need to change.
The Prime Minister has also said the UK wants more control over immigration when it leaves the EU.
Immigration is when people go to live or work in another country, so Theresa May wants to have more control over who can come to live and work here.
Being in the single market means you have to allow anyone from the EU to live and work here. As Theresa May says we are leaving that market, control over who comes here returns to the UK.
But Theresa May has said that she wants people from elsewhere in the EU who are living in the UK to be able to stay, and the same for people from the UK living in the EU.
Countries in the EU share some laws between them.
At the moment, British laws and European laws are closely connected.
If the UK leaves, it will need to decide which bits of EU laws it wants to keep for its own laws, and which bits it doesn't.
Given how connected EU and British laws are, it could take a while to separate them.
The UK might have to keep some rules if it wants to have certain relationships with the EU after leaving.
These are just some of the areas that the UK and EU will need to discuss and agree on.
The process for the UK to leave the EU could take even longer than two years, if all of the other countries in the EU agree it can be extended.
How long the process takes depends on whether or not the UK and the EU are able to agree on how their relationship will work.
If Article 50 is triggered, the UK will not be able to come back into the EU unless all the other 27 countries in the group agree that it can.