Often we use the phrase, "Knock on wood" - and proceed to do just that: Knock on a table or a door or whatever wood is handy.
In some circles it is believed that the origin of the phrase and practice comes from the wood of the rosary. Rosaries in the old days were made of oak wood and were fingered in time of distress or trouble. Thus, holding on to, touching or rubbing the wooden rosary or its wooden crucifix when danger was near became a common way for Christians to deal with hardships and difficulties.
The practice slipped into common use as "Knock on wood."