Question

The double-slit experiment with electrons illustrates the surprising idea that

Answer 1

Answer: electrons sometimes behave like waves

The French physicist Louis De Broglie proposed the existence of matter waves, that is to say that all matter has a wave associated with it.  

On the other hand, Heisenberg enunciated the uncertainty principle, which postulates that the fact that each particle has a wave associated with it, imposes restrictions on the ability to determine its position and speed at the same time.  

These postulations were tested with the double slit experiment (formerly applied to photons) applied to electrons, and the result was: electrons (as well as the other particles different from the photons) are able to behave as waves.