To manufacture Butane Hash Oil (BHO), users typically fill a tube with marijuana and then fill the tube with butane. The butane strips the Trichomes (THC) from the marijuana plant and the butane boils off at room temperature. After the butane boils off, a sticky yellow residue is left behind. This is the Hash Oil.
The problem is that butane is highly flammable. As butane is put into the tube, it leaks into the atmosphere and if near a source of fire such as a stove, or water heater with a pilot light, there will probably be an explosion.