Knock on Wood

After learning about why we cross fingers, I began to wonder what the origin is for "knock on wood." Some say it's because most cultures believed that spirits lived in trees. The Irish would knock on trees for good luck from the leprechauns. However, I believe it has more to do with a Jewish version back to the Spanish Inquisition of the 15th century. Jews seeking refuge from persecution fled to synagogues built of wood, and they devised a coded knock to gain admission. As a result, many lives were saved so it became common to "knock on wood" for good luck.