What is the pattern of light illuminating two fine openings, two slits?
Classical particle, like ping-pong balls, would just go through one of the two openings and form two little bunches on the far screen (right in the figure). Classical waves would interfere and form a pattern with maxima and minima (left in the figure).
The photons are quantum particles, with both the wave and particle nature, and arrive one by one, as particle would do but forming an interference pattern, as waves would do. This can be seen in the video, which is an actual experiment we have perform, "just to be sure" that quantum mechanics works.
When I was a student this was one of the most striking and illuminating experiment I have done, showing at once one of the most intriguing concepts of quantum mechanics, the wave-particle duality. Once more experiments can resonate and teach more than a thousand words!
Image recorded with a Princeton PRO EMccd, gain at 95, integration time per frame 10 ms, accumulation over 1000 frames (here shown in 3 sec).