Lasers can produce one or more photons when given a certain amount of electricity. The photon or photons come out of a very small hole within a well known period of time. The speed of light is known, so the time the photons appear on the screen can be predicted. When photons are produced one at a time, what shows up on the screen are individual spots of light. If photons were waves then we would expect them to spread out as they travel and wash across a wide area of the screen, but that never happens. If photons were particles then we would expect them to appear at two points on the screen connected to the laser through the two slits in the middle. But that is not what happens either.