One of the key bones of contention in the Brexit negotiations is the UK's only land border - the 300 miles which separate Northern Ireland (NI) from the Republic of Ireland (ROI).
The current situation is that, both the UK and the ROI being in the EU Customs Union and the UK-ROI Common Travel Area (but not the Schengen Area), the border is wide open. Goods and people cross with no formalities at all, known as a soft border.
When the UK leaves the EU it will also leave the Customs Union so it can set up trade deals with other countries, eg the USA. (Countries in the Customs Union have their trade deals negotiated centrally by Brussels.) That means, potentially, goods will not be able to pass without taxation, known as a hard border.
The ROI doesn't want a hard border with the North and as a member of the EU will scupper the UK's leaving deal if that is proposed. The EU have suggested putting a notional border in the middle of the Irish sea, ie keeping NI in the customs union but the rest of the UK out.
Unfortunately, splitting off Northern Ireland like this is completely unacceptable to the Democratic Unionist Party of Northern Ireland (the DUP), ie, the party whose 10 Members of Parliament are keeping Theresa May in power because she doesn't have a complete majority in parliament. The DUP want the UK completely united in all respects as the slightest concession on this is a step on the path to NI leaving the UK.
What to do?
Well, there is already a border where this problem has been solved: the 1,000 mile border between Norway and Sweden.
Sweden is a full EU member, single market, customs union, the lot. Norway is not an EU member; not in the customs union, but is a member of the single market. Officially this makes the border "hard", but in fact it is very much "soft" - cars pass without stopping.
In theory this means Norway could act as a Trojan horse for goods; eg cars could be imported into Norway from outside the EU without the EU-mandated 10% import tax being paid, and then moved into the EU via Sweden and sold on cheaply (ie, 10% cheaper.)
This is prevented by "rules of origin" laws, and spot checks using cameras on the roads which cross the border.
We could do the same with the NI-ROI border; officially hard, but with a soft touch. In fact we will probably have to, there isn't really any other way of fixing this problem.