The most exciting signings, the most improbable signings, are made when a club takes a star player from a rival.
And Manchester United have done it twice in the past two seasons.
In the summer of 2012, Robin van Persie swapped the Emirates for Old Trafford.
He was Arsenal's captain, the Premier League's top scorer. And it took the personal touch from Sir Alex Ferguson for Arsene Wenger to agree to the sale.
It was made even sweeter for United fans when the Dutch striker scored the goals to secure a record 20th title. Van Persie moved to United for the chance to win trophies, and his decision was immediately vindicated.
The case of Juan Mata is slightly different. He was Chelsea's Player of the Year for two seasons in a row but found himself on the periphery under Jose Mourinho. As such, he's come to United to play regularly.
But given the situation United find themselves in under new manager David Moyes, Mata's arrival feels more like the start of a rebuild rather than the final piece of the puzzle.
That was van Persie or, 10 years earlier, Eric Cantona.
United were denied the title in 2012 on goal difference. And when champions Manchester City failed to significantly strengthen, the deal to sign van Persie felt like a power shift back towards Old Trafford. And it was more than just symbolic that he scored the winner in the Manchester derby at the Etihad Stadium last season.
If van Persie was Ferguson's key signing in 2012, in 1992 it was Cantona.
That summer, United had finished second in the old First Division behind Cantona's Leeds United. Ferguson needed a striker to fire another title charge, and a chance phone call ended up with the Frenchman moving across the Pennines.
Five months later United won their first championship for 26 years. They won it again the following year, as well as the FA Cup, Cantona scoring twice in a 4-0 win over Chelsea in the final.
United have spent millions buying some of the best players in the world.
They broke their transfer record for Gary Pallister, Roy Keane, Andy Cole, Dwight Yorke, Juan Sebastian Veron, Rio Ferdinand and Dimitar Berbatov.
And while some—most even—have gone on to have successful careers at Old Trafford, it's only Cantona and van Persie who have felt like game-changers, signings that have shifted the balance of power.
Mata is like van Persie and Cantona in that he was acquired from a domestic rival at the peak of his powers.
But van Persie and Cantona were the final pieces of the puzzle and both won Premier League titles in their first seasons in Manchester.
Moyes will hope Mata is as successful at Old Trafford as van Persie and Cantona.
But if he is he'll have helped facilitate a new era of finals and silverware, rather than arriving as the icing on the cake.