You put a metal rod halfway through a portal and open another portal so that the end of the rod that went through the first portal touches the other end of the rod. If you weld the two ends of the rod together, how will you remove the rod from the portals? Assume that the portals cannot close if there is something in the way.
Since you can't put the rod through the first portal before opening the second one, both portals are already opened but spaced far apart and on movable surfaces so that once the rod is halfway through the first portal, the surface with the second portal is moved closer to bring the ends of the rod together.
A friend asked me this question and I've been thinking about this for a long time now. I apologize for not giving more details before.