On 23 June 2016, UK adults made a historic decision. They voted for the UK to leave a group of countries called the European Union (EU).
This is called Brexit, made up of the words 'Britain' and 'exit'.
The vote was over two years ago, but it is still being talked about a lot as the UK hasn't left the EU yet. It is due to leave the EU at 11pm on Friday 29 March, 2019. (Read on to find out more about why it takes this long.)
At the moment, UK and EU politicians are making all the final arrangements for Brexit to happen, as there is lots to sort out.
No country has ever left the EU before, so it's a significant moment in European history. Let's look at what exactly happened.
For many years, UK politicians have argued about whether it is better for the UK to be in or out of the EU.
Back in 2013, Conservative MP David Cameron promised that if his party won the next general election, he would let voters have their say on whether or not the UK should stay in or leave. In 2015, the Conservative Party won that election.
David Cameron became prime minister and dedicated a lot of his time to renegotiating the UK's relationship with the EU.
True to his word, in February 2016, he set a date for the vote to take place on Thursday 23 June, 2016.
The issue divided many people, as politicians and voters across the UK all chose which side they would choose - remain or leave.
David Cameron said: "The choice is in your hands, but my recommendation is clear. I believe that Britain will be safer, stronger and better off by remaining in a reformed European Union."
The UK has been a member of the EU for more than 40 years, so it's a big deal to leave it.
Not only that, but no country has ever left the EU before, so it was - and still is - completely new territory for everyone involved.
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 people had spoken and the result was declared - the UK wanted to leave the EU.
Once the result of the vote was announced, David Cameron - who didn't get the result he'd campaigned for - said he was going to resign, and so the UK needed a new prime minister before anything else could happen.
Prime Minister Theresa May took over the job the following month in July 2016.
Ever since then, the UK government has been trying to sort out what the UK's relationship with the EU will be like when it leaves.
The vote was over two years ago, but the UK is still a member of the group.
That's because the process for a country to leave the EU officially takes two years - and the process was only officially kicked into action on 29 March 2017.
This official moment that the process began was when Theresa May triggered Article 50 - an agreement between EU members about how a country can leave the group.
Nobody has ever put Article 50 into practice before - until now - so this was a significant moment.
Ever since Article 50 was triggered, the UK has been left out of making big EU decisions, but it still remains a member of the group and has to follow EU rules and agreements.
There are many things that have needed to be discussed and agreed upon before the UK can officially leave.
- Immigration and people - With around three million people from other EU countries living, working or studying in the UK, and around two million British people living in other EU countries, politicians needed to decide what would happen with those in countries where they weren't citizens. It also needed to be decided how immigration would work in the future between the EU and the UK once it is no longer a member, as people would not automatically be able to move across the border as freely as they could before.
- Buying and selling - One of the main reasons the EU was set up was to make it easier for EU countries to buy and sell products and services to each other. The UK has needed to work out how it will do trade with EU countries if it is no longer part of something called the single market, which makes doing this easier between member countries.
- Laws - Countries in the EU share some laws between them. At the moment, British laws and European laws are closely connected. The UK has needed to decide which bits of EU law it wants to keep, 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.
- 'Brexit bill' - The UK will need to pay the EU a sum of money when it leaves. This money will cover anything which we've already agreed to give funding for, as well as any ongoing payments for certain benefits once we've left. Politicians needed to agree what this sum of money would be.
- Irish border - As part of the UK, Northern Ireland is also due to leave the EU, but the Republic of Ireland will remain a member of the EU. Currently, many people move freely across this invisible border. Once Northern Ireland has left the EU, the border will need to be carefully managed, so politicians have needed to work out how they will do this.
...and these are just some of the many issues that UK and EU leaders have spent the last two years discussing.
The process of the UK leaving the EU could take even longer than two years, but only if all of the other countries in the EU agree for the timescale to be extended.
Mrs May has come up with a deal which she says is the best that the UK is going to get, which spells out what the UK's relationship with the EU will be like once it leaves.
On Wednesday 14 November, the cabinet - which is the prime minister's group of top ministers - agreed on the deal, although not all of them were happy about it, and some have since left their jobs as a result.
After 20 months of negotiations, on Sunday 25 November, the 27 EU leaders signed off on the deal too. They have insisted it is the "best and only deal possible".
Now, it needs to be approved by the UK Parliament. The UK Parliament is expected to vote on the deal on 12 December, but nobody knows if Parliament will approve the deal or not. Labour, the Lib Dems, the SNP, the DUP and many Conservatives MPs are set to vote against it.
Mrs May has said she is committed to seeing the process through and has urged Parliament to sign off on the deal too. She explained that the British public "do not want to spend any more time arguing about Brexit".
Now, we are waiting to see what the UK Parliament will do.