A ‘doublet’ is a word game in which you change one word into another word, a single letter at a time. For example, DOG → DOT → COT → CAT.
‘It is, perhaps, needless to state that it is de rigueur that the links should be English words, such as might be used in good society.’ (Lewis Carroll)
See if you can do it with SPIT and FIRE.
We did it in steps. Can you do better?
Congratulations! You have built your SPITFIRE!