How to I set the option to enable placing 1 solid part inside another?

Hi guys, I remembered that I used to be able to put 1 part inside another just by using the move option. Recently I can't seem to do it. if I try doing that, the part would just go to the other end of the part it's colliding with (Collision already set to false), hence I figured that 1 of my settings might have cause the problem. Can anyone tell me which is it? Thanks!