What is the secret of the marriage bed of Odysseus and Penelope?

The "marriage" bed or more formally known as the "bridal" bed, was the bed that Odysseus made with his bare hands, and he carved it into a tree that was growing through his house. Odysseus had taken years to build the bed, and it was a gift to Penelope, as a symbol of his love for her. When Penelope tells him the bed had been moved, he gets upset (which only Odysseus would do, because only he and Penelope even knew about the bed.) and this proves to Penelope that he really is Odysseus.

The marriage bed/bridal bed was very sacred to Odysseus and Penelope, and only the two of them knew about it. There was a tree that was growing through a small room in their house, so Odysseus built a beautiful bed (bridal bed) he carved into it flowers, and ivory, and built it all by his self and with his bare hands. Odysseus initially built his entire house around this tree, and when Penelope states that she moved the bed, meaning she cut it out of the tree, it infuriates Odysseus (which Penelope knew it would) and then she knows that it really is him. But the basic point of the bed is that only he and Penelope knew about it, so it was used as a test.