One of the biggest conundrums is the wave/particle duality of quantum systems as it is observed in two-slit experiments. The interference pattern seen in a two-slit experiment suggests that the quantum system is a wave. But what exactly makes us say that the quantum system also behaves like a particle? My partial answer is: On the screen behind the two slits we see individual, point-like marks very much like we would expect from a particle (imagine a little ball) hitting the screen, and certainly not from a wave front hitting the screen. With sufficiently capable detectors it can be shown that the energy transmitted/released while creating the mark is quantized, i.e. it cannot continuously made arbitrarily small, like could be expected from a wave amplitude. My question: Are there other points worth noting that suggest to say that the quantum system behaves like a particle? Is it correct to say that we don't know that it is a particle, we only make observations and interpret them? The question would then be: apart from the two observations above, are there other, different observations suggesting particle?