How many four-letter human body parts can you name? There are at least 21.

(Possible) Answers:

1. BACK
2. BONE
3. CALF
4. CHIN
5. FACE
6. FOOT
7. HAIR
8. HAND
9. HEAD
10. HEEL 
11. IRIS
12. KNEE
13. LUNG
14. NAIL
15. NECK
16. NOSE
17. PALM
18. SHIN
19. SKIN
20. SOLE
21. VEIN