pack pat mass ball
What do these four words have in common?
If you've got it, find two more words with this unusual (but easy to describe) property, and reply with just those two words.
Another clue - I'm pretty sure that there's no two-letter words that have this property, although an comes very close. I also haven't yet found any five-letter or longer words with this property, although there's no reason there shouldn't be (theoretically).
And thanks Nick for another shot, tot and roast. Now I've got another puzzle, to figure out what ball, pat, pack, tot and roast have in common. Hmmmm.
PS: I've now found 9 other words with this property (13 total), but some depend on some pretty obscure dictionary entries.