What is the real story behind saying 'touch wood'?

'Touch wood' and 'knock on wood' are used as a charm to bring good luck or to avoid 'tempting fate' after making a boastful statement. Some believe it has to do with knocking on the wooden cross. Another explanation stems from the pagan belief that spirits lived in trees and that knocking on wood would prevent the spirit from hearing the boastful words. The superstition has not been traced beyond the 19th century children's tag games.