5 Answers

I am with Pythonlover on this one.   ... the touching wood is to take away the jinx because you said it out loud....

I haven't heard of that one,i thought it was, if you touch wood, it is in the hope that what you said doesn't come true.

                             e.g    I hope i don't get the Flu this winter.(Then you touch wood).

ancient belief is that spirits dwelled in or guarded  trees around the world shows evidence of tree worship dating back thousands of years.

 greeks worshiped the oak as it was sacred to zeus.celts beleived in tree spirits and touching sacred trees would bring good fortune pagans also held similar beliefs of protective tree spirits chineese and koreans thought the spirits of mothers who died giving birth remained in nearby trees.

 another explantion point to the wooden christian cross as the origan of good luck.although this is likely an adaption of  earlier pagan practices.

a jewish version traces the origin to the spainish  inquistion of the 15th century, at the time persecuted jews  fled to synagogues built of wood, and they devised a coded knock to gain admission. since this practice saved countless lives , it became common to knock on wood for luck..

Answer: 

When children play tag and hold a tree for safety, they are acting out a four-thousand-year-old custom of the North American Indians. who considered that because the oak was most often struck by lightning, it was the home of the sky god.  The Greeks, as daren1 alluded to above, came to this same conclusion two thousand years later and because both cultures believed that bragging or boasting offended that god, they knocked on the tree either to divert him from their bragging or to seek forgiveness.

For this, daren1 has the best answer.

I'm not superstitious knock on wood... :)