It's got a basket, a bell that rings and things to make it look good.

An early orb sighting, from about 1500!
in 1961, the art historian Carlo Pedretti held a page of drawings by Leonardo da Vinci up to the light to see what was on the back. He saw ORBS.
Subsequently, the disordered portfolio of sheets to which this page belonged -- the Codex Atlanticus -- was exposed to a process of amateur restoration by a cloister of monks (who were bored with distilling liqueurs and had asked the Vatican for something else to do). This new hobby occupied them from 1966 to 1969.

In 1974, the now-exposed back of the page looked like this, now including additional lines in different-coloured ink that link the circles into a crude bicycle design:

It is patently a COVER-UP to CONCEAL THE REALITY OF ORBS.

The cover-up inspired Martin Gardner to reveal in his April 1975 Mathematical Games column that Leonardo had also invented the flush toilet, but he may have been joking.

Not everyone was convinced by the "Leonardo's Bicycle" thesis. A rival theory (Bimler, 1995) points out that bicycles are frequently confused with glasses, so the diagram may actually represented the latter.

In 1997, the 8th International Cycling History Conference (in Glasgow) was collectively dumbstruck when Professor Strabismus (whom God preserve) of Utrecht Professor Dr. Hans-Erhard Lessing of Ulm presented his seminal paper The Evidence against 'Leonardo's Bicycle'. If you couldn't attend, never fear, thanks to our helpful research interns you can read it here. Or more here.

Alternatively, the Great Gazoogle reveals that any number of people still take the story seriously. I particularly like the argument that Leonardo "looked into the 'Philosopher's Stone' of his patron/alchemist or he developed one of his own like Nostradamus. [...] If he actually traveled through time he would have been able to draw a bike and many other things like his famous helicopter."

No-one mentions the Mollycule Theory or the dangers of atomic interchange.