In a thought experiment, there is a spin-0 source emitting particles. (I suppose you already know what is that experiment.) Two observers in opposite sides along the same axis measure opposite spin components. If one observer measure, say, spin up, then the second observer will certainly measure spin down. Then it's said that the measurement by one observer affects what the another observer will find. But why don't conclude that the system (composed by the two particles) was already set up for that particular pair of results (spin up to one particle and down to the other particle) before the first observer interact with the system?