preety simple really, as karsonleverit said, if you don't inflate the tube before fitting the tyre, the tube is then seated under the bead of the tyre, when you inflate the tyre to a relatively high pressure the fecking thing explodes, or if the tyre doesn't seat properly on the rim when inflated a part of it pops of the rim exposing the innertube, the it only takes a liitle nock on a sharp/rough surface (wall maybe?) to pop the thing.