"Touch wood" or "knock on wood" is a popular saying to reverse any bad luck that might come your way, due to a recent action you have taken or comment you have made. The superstition is that if someone remarks about something not wished for, or something that has not happened lately, they should immediately touch or knock on wood to avoid the fulfillment of bad luck, or the event not desired. Touching wood after boasting of one's good fortune, or exemption from bad luck, was thought necessary to keep the streak of good luck intact. Still, today, people are likely to touch wood immediately after answering a question in the negative, such as "no, I have not been sick lately."
Touching wood is also practiced after an activity or event that has gone excessively well. As an example, if you give a good presentation you should touch wood afterwards to continue the success you are experiencing. People will often go to great lengths to find a piece of wood to touch after one of the events described above have occurred. They believe that if they do not touch wood, the fates will wreak vengeance upon them.
Touching wood is also a superstition practiced to ensure safety and good fortune at the start of a day or activity. Athletes and actors are among two of the professions that practice this superstition to increase the good luck they will have in their upcoming game or show.
Touching iron used to be popular as well when iron was more prevalent and thought to hold magical properties. The origin of the use of wood is thought to be associated with the cross that Jesus was crucified upon. Smaller crucifix pendants were made and worn as good luck charms, often made from wood. Wood then began to be associated with safety, magic and good fortune.