An oxy-propane torch is lit with just propane, and the flame examined, before oxygen is added, changing the flame character and temperature.
The torch head is gaffed to the table top with enough slack to turn the gas valves. The propane tank and the oxygen tank are on separate dollies. The torch valves are closed, and the tank regulators set so there is about three pounds of pressure behind each gas. (The precise settings of the regulator are a good thing to check out in a practice session before doing this demo.)
With a friction lighter in one hand, open the propane valve on the torch a titch, and light the gas. Increase the flow until the rich, yellow flame is about 20 cm long. Admire.
Slowly open the oxygen valve, and watch the flame change, getting more intense yellow, then blue with tinge of yellow, then blue, then screaming blue. Turn the oxygen back down to a neutral flame, and turn off the propane at the torch to get the mixture to bark as the flame goes out. Turn off the oxygen at the torch, and put it on the bench.
Close the main valves of the propane and oxygen tanks, vent the regulators, close the outlet valves, and back off the regulators. Disconnect the quick-connects on the gas lines, and remove the cylinders to the prep room.