Abstract: The invention provides a plug for sealing substances in holes drilled in wood. The plug has a body of generally circular cross section and a thread portion helically wound around the body between inner and outer end portions thereof. The thread portion has a leading face facing towards the inner end portion and a trailing face facing towards the outer end portion, the leading face being inclined at a shallower angle to a central axis of the body than the trailing face. The plug can be driven into the hole by hammering, but is extracted from the hole by unscrewing in a normal manner. The thread portion is designed to permit relatively easy movement into the hole without rotation, but requires rotation for extraction from the hole. Preferably, the thread portion is flexible and the thread portion is within a shallow conical periphery. The outer end portion has a recess to receive a complementary driver to rotate the plug for extraction.