Bell's Theorem

A particularly nice interactive explanation can be found here, and also here.

The best explanation for Bell's theorem in layman terms, can be found in The Quantum Theory and Reality by d'Espagnat

Another nice explanation can be found in this reddit comment.

Another creative explanation in laymen terms can be found in The mystery of the quantum cakes by P. G. Kwiat and L. Hardy

A simple proof of Bell's inequality by Lorenzo Maccone

http://mateusaraujo.info/2016/07/15/understanding-bells-theorem-part-1-the-simple-version/

Bell's Theorem with Easy Math by David R. Schneider

See also David Mermin's paper, "Bringing home the atomic world: Quantum mysteries for anybody" Am. J. Phys. 49:940-943(10), Oct. 1981, free PDF.

Daniel M. Greenberger, Michael A. Horne, Abner Shimony, Anton Zeilinger "Bell's theorem without inequalities", American Journal of Physics Vol. 58 (12), December 1990

For another nice discussion, see What could not be in Feynman lectures: Bell inequalities by V. Cerny