Confident EU coy on start date for Brexit trade talks
So how long is that famous piece of string? I certainly don't know.
Nor, I suspect, does the European Commission. Or the press. Or the UK government.
So, trying right now to answer the vexed question (for the UK) as to when exactly, during Brexit negotiations, the time will come to turn attention from divorce to that much anticipated new EU-UK trade deal is possibly rather futile.
As we know, the EU's draft guidelines for negotiations state that talk of the future will only begin in earnest when good progress has been made on Britain's exit deal.
But when, and based on what criteria?
The only thing we know for sure is that it is in the EU's gift to make that judgement. Not the UK's.
First Vice-President of the European Commission Frans Timmermans told me point-blank there could be no agreement on the future "if we're not very clear what the divorce settlement is going to look like".
So if the UK wishes to see through the process of making that free-trade deal, it will have to wait for Brussels to switch on the green light.
"Not dissimilar to the Greece conundrum," an EU diplomat commented to me this weekend.
The EU has told debt-laden Greece it will only countenance debt relief once Athens has made sufficient progress on restructuring and reform.
As with the EU conditions for UK trade talks, Greece finds itself staring at unquantifiable strands of EU string.
The mood in Brussels right now is cautiously bullish (an interesting state of being).
Of course Brexit hurts. The EU has lost one of its influential members, a big contributor to the EU budget, a powerful economy, and one of only two serious military powers in Europe (France being the other).
But as soon as Article 50 was triggered last week, the words of sadness and regret that poured out of Brussels following the UK's EU referendum vanished into the mists of Dover.
Tables turned on Gibraltar
"Britain is now on the other side of the negotiating table," said European Council President Donald Tusk on Friday.
And the rest of the EU is closing ranks. Just look at the current row over Gibraltar.
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When Britain was on the same side of the table as the EU, Brussels remained resolutely neutral.
It was a diplomatic coup for Spain to have it written into the draft EU guidelines that any Brexit deal could only apply to Gibraltar with a nod from Spain (which contests British sovereignty over the territory).
These are only draft guidelines; they carry no legal weight and they still need to be formalised at a summit of the 27 remaining EU member countries on 29 April.
But this was a clear Brussels message: we look after our own.
A missive directed not only at Britain but, significantly and purposefully, at the remaining EU countries.
Across the Channel, Brexit is not just about the UK, but about safeguarding the European Union.
It's common knowledge that this is a fractious union, whose members fall out over funding, euro rules, migration and more.
It is also a common assumption, as Frans Timmermans put it, that each side in a negotiation seeks out the other's weak spots.
The UK - respected and feared in Brussels as a wily negotiating power - is expected to try to divide and rule in the EU during the Brexit process by promising individual countries custom-made sweetheart deals.
Security against Russia for the Baltic States, peace of mind for Poland and Spain's citizens in the UK, an Irish border deal that doesn't harm the Good Friday Agreement and so on, in the hope those countries will champion the UK against any hardline attitude from Brussels.
But the EU needs to unite to survive and maintain credibility after the UK walks out of the door.
"However much we want and, to be honest, we ideally need a good future relationship with Britain, it has to be clear we're there for the EU 27 now," one high-level Brussels source told me.
The EU's draft Brexit guidelines were designed to be firm-sounding towards Britain - a warning for others who may want to leave the EU - but they had a plethora of priority pledges for those who stay: Ireland (land border), Spain (Gibraltar), all those countries with citizens living in Britain, nations with security fears and businesses with logistical concerns post-Brexit.
EU gains self-confidence
Easy promises, of course, before negotiations begin but the EU, for the first time in many, many months, is feeling less beleaguered, less on the back foot; more confident.
While an undeniably huge blow to the EU, Brexit has served to concentrate Europeans' minds on what membership means to them.
Germany's Süddeutsche newspaper on Monday had a front-page photo and article on the pro-EU demonstrations it says took place in 60 European cities this weekend, organised by the Pulse of Europe initiative.
The activists' aim: to stop the EU debate being dominated by the voices of Eurosceptic nationalists across the continent.
Brussels was already buoyed by the success of unashamedly pro-EU parties in last month's elections in the Netherlands.
It has hopes, too, for French presidential candidate Emmanuel Macron and looks contentedly at the two pro-EU front runners in Germany's upcoming elections.
News on growth in the eurozone is comforting for Brussels, which prefers to overlook the fault lines in Greece and Italy when possible.
And the latest EU feel-good factor comes from perhaps the most surprising source of all: the 45th President of the United States, Donald Trump. The man who, weeks ago, publicly prophesied that other EU countries would likely follow the UK's example and leave.
Yet, in an interview published in Monday's Financial Times, President Trump appears to have changed his mind. In fact he is quoted as saying he thinks the European Union is getting its act together.
But even the most ardent of Euro-enthusiasts admit maintaining EU unity during divisive Brexit talks will be tough. Never mind all the other challenges the bloc currently faces.
"We (in the EU) can't be naive," European Commission President Jean-Claude Juncker said to me just before last week's triggering of Article 50.
"This is no time for complacency."