The classic one is "can you hook up someone to be the dialysis machine of someone else against their will"? Even if foetus are people, they shouldn't have more rights than other people.
But since cruelty is the point (punishment for having sex) there's no actual reasoning with them.
The concept of a glass being full and of a liquid being wine can probably be separated fairly well. I assume that as models got more complex they started being able to do this more.
But then you see them inverted.
3d cross eyed pictures and magic eye work in similar but different ways