wifreo Perfume Essential Oil Bottles, Square Refillable Thick Glass Roller Bottle, 8 Pack 1/3oz 10ml

Is there a trick to using the opener? It seems the rollerball fits tight but after I open it and put it back it is loose and then leaks.

How do you open it? I’ve been twisting & twisting & can’t get it open…

Can this be used for a thick oil like cold pressed castor oil to roll on the scalp??

What is the millimeter size of mouth opening?