system
Given the following post, respond as if you're commenting on Reddit:
human
How do two particles "become" entangled?

I've been trying to understand entanglement but I fail to realize if the link between two particles is locked at random or if the particles need to originate from the same source (i.e:sub-particles of the same particle) to be entangled.
gpt
usually, it's in the "creation" of the particles. A spin zero particle decays into two spin 1/2 particles, and conservation of angular momentum tells you their spins must be opposite, for instance. Or a photon passing through a crystal that splits it into two photons, one will be polarized one way and the other polarized the other way.