What peculiar property do all these words have in common?
BE CONTENT CRED DO EVENS FIRS HE INNS MUSS RATS RED SHILLS SLED SLY SPRY STAGS STRAY STY TAPS WED
I would have liked to have included RAIN and RATE because they're so much fun, but I anticipated someone would contest them on a technicality: the –ING– gets put at the beginning, not stuffed in the middle.
Here are some others that didn't make the shortlist: BED, DOES, FRY, MATS, SWARM.
There is also an entire family of words like SING ––> SINGING in which there could be ambiguity as to whether the additional –ING– has been put at the end or stuffed in the middle.
BINGING, BRINGING, CLINGING, CRINGING, DINGING, FLINGING, FRINGING, HAMSTRINGING, HANDWRINGING, HINGING, IMPINGING, INFRINGING, MUDSLINGING, PINGING, RINGING, SINGING, SLINGING, SPRINGING, STINGING, STRINGING, SWINGING, UNHINGING, UPBRINGING, UPBRINGINGS, WINGING, WRINGING, ZINGING