Quantum entanglement
Also, without sending information about the partner particle, there's no way to tell if a given particle is single or half of an entangled pair. With no outside information, a single particle is completely like any other. It's only when you can receive information about the other particle that you will be able to figure out if your particle is one part of an entangled pair. No one can use entanglement to send information faster than the speed of light because you would need another faster than light communicator to do it.