Nannerl later recounted that when he was just four years old he took great pleasure in picking out thirds on the keyboard and playing simple tunes.
Mozart’s father, Leopold, seeking to hone his childrens’ talents (and probably to make a little money) began having them play for family and friends, many of whom were courtiers and aristocrats. As well as regular performances, the pair would perform tricks - Mozart would play blind with the keys covered by a handkerchief, for instance.
Word of the Mozarts’ popularity spread quickly, and in 1762 they were invited to perform before the Empress Maria Theresa - an aunt of Marie Antoinette.
The Empress, famously stoic, was visibly delighted at the childrens’ playing. A popular story from the visit describes how Mozart leapt into her arms and kissed her on the cheek!
With the reputation from their performances spreading far beyond Austria, Leopold decided they would capitalise on it by organising a “grand musical tour” of Europe.
The family travelled to more than six countries in just three years, sometimes performing several times daily and paying their way with the fees.
This trip is often credited as the source of Mozart’s great talent for composing (which, aged seven, he was just beginning to experiment with). He was introduced to many different musical styles and studied under many famous teachers, leading to a highly eclectic style.
Mozart wrote his first symphony in 1764 while staying in London - partially to make a quick buck, as his father had fallen ill, putting a stop to perfomances. (You can listen to it in full here!)
Mozart’s writing continued prolifically for many years, and although he remained a skilled performer he became more known for his tunes. His tour of Europe had given him a love for many different kinds of music, and he reflected them all in his writing - from his bawdy, comedic operas to his religious music, often grandiose and serious.
Hardly surprising that now, almost 300 years later, the little keyboard-playing kid from Salzburg is and remains one of the most popular composers of all time.
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It’s sad to note that while Mozart’s star grew brighter, Nannerl’s did not. A few years after the end of the tour, her parents’ conservative views led to her all but giving up music entirely and settling into what was then the norm for marriageable women. She and Mozart remained close friends throughout their lives, however, and much of of their correspondence (in the form of letters) has been documented. A lot of it is quite sweet, and you can actually read most of it online here at Project Gutenberg.