How does Nmap detect operating system?

Related questions

1. What Nmap flag is used for OS detection?
2. What is port scanning used for?
3. Why do we use nmap?
4. What are the types of scanning?
5. What port does Nmap use?
6. What is Nmap used for?
7. What is an ACK scan?
8. How does nmap do OS fingerprinting?
9. Why is nmap used?
10. What is nmap stealth scan?

Tags

1. #operating
2. #scanning
3. #analyzes
4. #sampling
5. #systems
6. #internal
7. #system
8. #tests
9. #sends
10. #response
11. #compares
12. #database
13. #detect
14. #variety
15. #device
16. #packets