Final answer:
Before using the cutting lever on an oxy-acetylene torch, one must wait for the steel to become cherry red, which shows it has reached the required temperature.
Step-by-step explanation:
When cutting with an oxy-acetylene torch, one should observe a change in steel color to a cherry red before depressing the cutting lever.
This indicates that the steel has been preheated to the kindling temperature needed to support the exothermic reaction that will continue the cutting process.
The heat from the torch melts the metal, and the lever allows a stream of pure oxygen to blow the molten metal away, creating the cut.