> how you can be sure about the state of the other particle, what if someone already measured it and then did something to it? Indeed, you are only sure about

> how you can be sure about the state of the other particle, what if someone already measured it and then did something to it? Indeed, you are only sure about the state of the other particle in the instant just after they measured it. Whoever measures first instantly destroys the entanglement link, so if they chose to manipulate the particle after measurement, you will have no knowledge of these manipulations. More generally, note that in quantum mechanics "reading" the state of a particle (i.e. performing a measurement) is drastically different than "writing" information by manipulating a particle. Most entanglement-related weirdness hinges on this fundamental asymmetry between "read" and "write" operations for quantum information.