Impossible screw

The magician holds a double-headed screw with two threaded studs of different sizes: one larger and the other smaller. He then screws a nut onto the small stud and then onto the larger one... the IMPOSSIBLE part is that he always uses the same nut! At the end, he gives the two pieces to a spectator and asks them to do the same... obviously, they can't do it!

• No skills are required, it can be repeated immediately.

• The object is made of brass, everything can be examined.

• When the nut is screwed onto one of the two studs, the screw is 4.5 cm (2 inches) long and has a diameter of 1 cm.