What is going on in British politics? A guide for the confused
If a butterfly flaps its wings in... well, who knows where all this started.
But the people of Britain - that sensible democracy, so we thought - now have good reason to be acquainted with chaos theory. Or at least that bit of it that says small changes in initial conditions can lead to dramatic changes in results.
As of today (and we have learned things may be very different tomorrow), the two major political parties are changing or trying to change their leaders and the country is about to get a new prime minister.
How can you have a new PM without an election?
British prime ministers are not directly elected - they become head of government by virtue of leading a party that can command a majority in the House of Commons.
So when the party leader, in this case David Cameron of the Conservatives, resigns, his successor automatically becomes prime minister.
In some cases, a new leader may decide to seek their own mandate by then calling a general election. But there is no requirement to do so.
Some may remember that Labour's Gordon Brown didn't go to the public after succeeding Tony Blair in 2007.
And the Fixed-term Parliaments Act of 2011 now makes it more difficult to have an early election.
This means new Conservative leader Theresa May could govern until 2020 before a general election is held.
What are PMQs and what did Mr Cameron say at his last session?
Mr Cameron's final, or nearly final, act as prime minister was to appear in the Commons for Prime Minister's Questions (PMQs).
This is the weekly opportunity for MPs to throw verbal brickbats or bouquets. Friendly ones lob gentle questions while others hold the prime minister to account and seek to land blows.
In this adversarial environment, there's also the chance for mockery, jokes and insults.
Mr Cameron appeared to enjoy his last appearance, mixing answers to policy questions with light-hearted moments, good wishes for MPs on all sides and an admission that he would miss "the roar of the crowd".
He won't be going too far, though. Although stepping down as PM, Mr Cameron will remain an MP and intends to stay in public life.
Where's the Queen? And what's her role?
The Queen, who was at her country residence at Sandringham, is now back in London and has an important constitutional role to play.
Mr Cameron will go to the monarch to hand in his resignation. Shortly afterwards, Mrs May will arrive at Buckingham Palace where she will be invited to form a government.
Following tradition, she will "kiss hands" with the head of state - in reality, shaking hands.
Will Mrs May move in to 10 Downing Street tonight? And where will David Cameron go?
Things move fast after a prime ministerial resignation. The removers have already been in to 10 Downing Street and Mr Cameron, his family and staff will be leaving straight away.
Mr Cameron may return to his London home several miles away, where the tenants have been given notice, or he may go back to his constituency home in Oxfordshire.
His staff will be seeking new jobs. After the 2015 election, Mr Cameron said he would not serve as PM beyond 2020. After the referendum, he announced his plan to step down in September.
But events have moved more quickly, and now those who thought they had at least two months to find other employment will be out of work.
Mrs May will arrive in Downing Street as prime minister on Wednesday evening, though it may be a little longer before her household stuff follows.
It's not known if she has sought Mr Cameron's advice on the job, but some Twitter users have had fun imagining the conversation between them.
Who gets custody of Larry the cat?
Larry the cat (a fixture in Downing Street but not owned by the Camerons) will retain his position as prime minister's chief mouser, we are assured.
Mr Cameron has done his best to scotch suspicions that he is not a cat lover. Before going to Parliament for the final time as PM, he tweeted this picture for posterity.
Since Larry arrived in Downing St in 2011, the leaders of the Conservatives, Labour, Liberal Democrats, Greens, Scottish Nationalist Party (SNP) and UKIP have changed. That makes the Downing St cat one of the great political survivors of our times.