If I have a bottle of salt water and put it in place of rotor in induction motor, will the salt water spin ? Why / why not ?
Probably yes. Any small amount of torque would make a liquid spin. Eventually it could approach the angular frequency of the applied field. Of course it depends on conductivity, it should be easier with mercury than with salt water.
And actually, here is a reference from 1965: On fluid flow induced by a rotating magnetic field. That paper has since been cited a lot in magnetohydrodynamics studies. Here an article in fulltext with a good introduction: https://perso.limsi.fr/witko/publis/ejm98.pdf