“the Double-Slit Wave/Particle Duality Experiment”
A photon (or electron) is fired at Two Slits; an interference pattern (stripes) is revealed on a screen on the other side.
“Some scientists say” that the photon (or electron) travels through both slits simultaneously, before “deciding” [metaphorical use of the word] where it will end up on the screen.
Scientists also say that when the photon (or electron) travels through both slits “probabilistic mathematics happens”.
The seeming duality of the photon/electron behaving as a discrete unit (particle) and as elastic energy (wave) is said to be a paradox.