The burning sensation produced by garlic is caused by a compound called alliin, the when exposed to the air is converted to allicin. The allicin then interacts with transient receptor potential ion channels in the mouth and on the tongue. This channels are a group of molocules that react to temperature and when stimulated are intepreted as pain. But the garlic causes no physical damage to the tissues, so the sensation of burning is interpreted but no actual burn ing will take place. The allicin is also quickly converted to a non active aubstance so the effect is transitory.