3-letter words:
* hum
* rum
* our
* sum
* us
4-letter words:
* homo
* moor
* mush
* sour
* sumo
5-letter words:
* humor
* mouse
* rouse
6-letter words:
* rumour
*
7-letter words:
*
8-letter words:
* humorous
This is not an exhaustive list, but it gives you a good starting point. There may be other words depending on your interpretation of the rules!