Entanglement

When 2 particles interacted and they become correlated, or mathematically, their wavefunctions are intertwined and become one wavefunction in superposition, any change (e.g. collapse of wavefunction by taking a measurement or one of the wavefunction encountered a double-slit) to one of the particle will have a non-local effect to the other instantaneously, no matter how far the 2 particles are apart in distance.