How do I get a fluid to pour into an cup?

I’m dropping a fluid into a cup and I’m having trouble getting it in. No matter the resolution it never goes in. It acts as if the cup’s top were a solid face.

I heard it may have something to do with normals?

Change your obstacle type from Volume to Shell.

I’m looking at the preview and that did the trick.

I swear I never seen a fluid tutorial that emphasized volume and shell. Thank you for answering my simple problem.