A theory, that is on a spherical body, if you move forward on a straight line, you will find that you will move back to your starting position.

In this theory, it is said that it has limit space but have no border.

Limit space is the volume of the body was fixed, you cannot change it.

If you have a huge ball that able to let you walk on it, you will find that, wherever you walk, you will never find its border.