An apple - although it was an observation of an apple falling. . Just for interests sake - there is an apply tree on the campus of Cambridge University, England, where Newton was on the faculty, that is said to be the offspring of the famous apple tree at his home in the country, where he was staying at the time to escape the plague.. . Since someone gave me a "thumbs down" here is some additional information from multiple sources:. . Newton and the Apple. . The story that Newton's hypothesis of universal gravitation was prompted by the fall of an apple, is probably based on fact. There are several reliable accounts from the last few years of his life which record him describing such an event. (There is no indication an apple fell on his head though...) . . . The first was recorded by William Stukeley, who said that after dining with Newton in Kensington on 15 April 1726: . . . The weather being warm, we went into the garden and drank tea, under shade of some apple-trees, only he and myself. Amidst other discourses, he told me, he was just in the same situation, as when formerly, the notion of gravitation came into his mind. It was occasioned by the fall of an apple, as he sat in contemplative mood. Why should that apple always descend perpendicularly to the ground, thought he to himself. Why should it not go sideways or upwards, but constantly to the earth's centre. . . . From Memoirs of Sir Isaac Newton's Life pp19-20. . . . John Conduit recorded a similar story: . . . In the year 1666 he retired again from Cambridge ... to his mother in Lincolnshire & whilst he was musing in a garden it came into his thought that the power of gravity (which brought an apple from the tree to the ground) was not limited to a certain distance from the earth but that this power must extend much farther than was usually thought. Why not as high as the moon said he to himself & if so that must influence her motion & perhaps retain her in her orbit. . . . Keynes Collection, King's College Cambridge, MS 130.4 pp10-12 . . . There are also two versions published by Voltaire in the eighteenth century. In the Essay on the Civil War in France(1727) he stated that Sir Isaac Newton walking in his Garden had the first thought of his System of Gravitation, upon seeing an Apple failing down from the Tree. In the Letters Concerning the English Nation(1733) he referred instead to a fruit falling from a tree and that he had this story from Newton's niece, Catherine Barton who was also Conduitt's wife. It should be remembered that Voltaire was in London for the last year of Newton's life and for two years after and met several of his friends - although he got no closer to Newton than his funeral. . . . The story is also repeated by Robert Greene in his Principles of the Philosophy of the Expansive and Contractive Forces(1727, p972) who cites Martin Folkes, the vice-president of the Royal Society. . . . The tree shown to inquisitive visitors to Woolsthorpe Manor in the eighteenth century no longer survives. However, various saplings grow in Newtonian sites around the world, including the Babson College Library and on the lawn to the right of Great Gate of Trinity College Cambridge, in the garden Newton once owned. . . . Both the saplings and the apple trees currently growing at Woolsthorpe are of the species Flower of Kent, a pear-shaped cooking variety. . . The incident is also discussed by Richard de Villamil in his Newton: the Man.. . I would include the photo that I took of the above mentioned tree on the Cambridge University campus in 2004 but an image cannot be pasted into the answer field.