In the standard double-slit experiment, a wide screen is shielded from an electron gun by a wall containing two separated slits. If the electron gun is fired with one slit closed, a mound of electrons forms on the screen beyond the open slit, trailing off to the left and right — the sort of behaviour expected for particles. If the gun is fired when both slits are open, however, electrons stack along the screen in comb-like divisions. This illustrates the electrons interfering with each other — the hallmark of wave behaviour. When you see evidence of both waves and particles, the simplest explanation there could be is that both waves and particles are present. Similarly if you're on safari on the Serengeti and you see lion dung and elephant dung on the ground, you assume both lions and elephants have been around; you do NOT assume that some magical creature with properties of goth elephants and lions has been around.