Entanglement, by general consensus of physicists, is the
weirdest part of quantum science. To say that two particles, A and B, are
entangled means that they are actually two parts of an inseparable quantum
thing. An important consequence of this inherent kinship is that measuring a
property of A (say, the particle’s polarization) is necessarily to know the
corresponding property of B, even if you’re not there with a detector to
observe B and even if (as explained below) the existence of that property had
no prior fixed value until the moment particle A was detected.