Acorn and Oak
In Norse folklore, both the Acorn and its bearer, the oak tree, bring good fortune. The Vikings associated oak trees with Thor, the god who created thunder and lightning with his great anvil and hammer. Because the tree attracted lightning, it was sacred to Thor. Thus they believed that the Acorn, the fruit of the oak tree, was always spared the god's wrath, and so they began putting a lone Acorn on their windowsills to protect their houses from lightning's wrath.