> Now, to be fair, neutrino-mixing in and by itself isn’t all that weird. Indeed, quarks also do this mixing, it’s just that they don’t mix as much. That *neutrinos mix is weird because neutrinos can only mix if they have masses. But we don’t know how they get masses.

Could someone explain to a layman why "neutrinos can only mix if they have masses"? Perhaps I don't understand what it means to "mix"