I am really having trouble with understanding the experiment John Bell proposed to prove that the (spin) of one of the entangled particle pairs was indeed undecided until the moment the others orientation was measured.

This is how I understand it so far....

Two particles are created from a particle with spin 0, the particles need to have opposite spins to cancel out so as to conserve overall spin value. Both particles are sent in opposite directions, even miles away. The spin of one of the particles is measured and so the spin of the other particle is known because it has to have opposite spin.

Einstein used this experiment to demonstrate that the spin of both particles where decided the moment they where created "They had to be" but

John Bell put a second detector after the first and somehow proved that the spin of particle A was not actually decided until particle B was measured.

Could anybody explain in as simple terms as possible (cos Im not very clever) how this modification proves entanglement.

Many thanks

http://en.wikipedia.org/wiki/Bell's_inequality

 

http://en.wikipedia.org/wiki/EPR_paradox