M cannot replicate the particle after they read it, so A and B will detect M’s attempt when they compare results.
The same as classical one-way encryption, it only works through authenticated channel tho. It’s not magic, you have to have some kind of pre-existing secret or rely on third-party authentication
We gotta preserve our curiosity and childish naivite. Imagine yourself a professor sharing a beauty of math – the good ones are always as excited to explain it, as the first time they’ve felt it ^^