Why do they say “knock on wood”?

The phrase "knock on wood" is believed to have originated from a superstitious practice of touching or tapping on wood to ward off bad luck. It is thought that this practice dates back to ancient pagan cultures, who believed that spirits lived in trees and knocking on them would appease the spirits and bring good fortune. In some cultures, it was also believed that knocking on wood could summon helpful spirits.

Today, the phrase "knock on wood" is used as a superstitious way of protecting oneself from bad luck. By knocking on wood, a person is asking for protection from whatever misfortune they may be worried about. It is also believed to be a way of thanking the spirits for protection. In some cases, people may even knock on wood twice, to ensure that the good luck will last.