Your blowpipe is a tube. Hot glass covers one end of the tube. The laws of physics state that gasses will expand when heated. If you pressurize the air in the tube (using your breath) and seal it with your thumb, the glass on the end of the pipe will seek the path of least resistenace to escape. If the glass is hot enough, it will expand there to inflate and make a bubble. If your thumb slips off, the pressurized air will escape and your bubble will cease to expand.