I have these tables

Boxes:
id | name
1 | oranges
2 | mangoes
3 | Bananas
4 | misc

thingsinboxes:
id | box_id | thing
1 | 1 | a
2 | 1 | b
3 | 2 | c
4 | 3 | d
5 | 3 | e
6 | 4 | a
7 | 4 | b
8 | 4 | c


How do i get thingsinboxes.box_id that has thing='a' AND thing='b'

i dont want anything returned when i just say thing ='a' because i am leaving out thing='b'