When Mozart was eight years old, Barrington observed him in a sort of experiment. The results were published in the Philosophical Transactions of the Royal Society in 1771.
“I said to the boy, that I should be glad to hear an extemporary Love Song … The boy … looked back with much archness, and immediately began five or six lines of a jargon recitative proper to introduce a love song [in an opera]. He then played a symphony which might correspond with an air composed to the single word, Affetto. It had a first and second part, which, together with the symphonies, was of the length that opera songs generally last: if this extemporary composition was not amazingly capital, yet it was really above mediocrity, and shewed most extraordinary readiness of invention. Finding that he was in humour, and as it were inspired, I then desired him to compose a Song of Rage, such as might be proper for the opera stage. The boy again looked back with much archness, and began five or six lines of a jargon recitative proper to precede a Song of Anger. This lasted also about the same time as the Song of Love; and in the middle of it, he had worked himself up to such a pitch, that he beat his harpsichord like a person possessed, rising sometimes in his chair … [H]e had a thorough knowledge of the fundamental principles of composition, as, upon producing a treble, he immediately wrote a base under it, which, when tried, had a very good effect. He was also a great master of modulation, and his transitions from one key to another were excessively natural and judicious; he practiced in this manner for a considerable time with a handkerchief over the keys of the harpsichord.”
To learn more about Mozart as a musical prodigy, take a look at Kevin Sullivan’s documentary, Mozart Decoded.
Source: The Treasures of Mozart by John Irving