Why NoSQL databases are horizontally scalable?

Related questions

1. What is scalable network?
2. What is scalable solution?
3. What is Web scaling?
4. What does scaling horizontally mean?
5. What applications are scalable?
6. Does vertical scaling improve availability?
7. How does scalability work?
8. What is scalable code?
9. What is difference between scale up and scale out?
10. What is scalable pricing?

Tags

1. #example
2. #sharding
3. #mongodb
4. #support
5. #databases
6. #scalable
7. #nosql
8. #considered
9. #between
10. #scalability
11. #built
12. #partitioning
13. #horizontal
14. #differences
15. #replication