Why is MongoDB considered flexible in its schema?

MongoDB is considered flexible in its schema because a document's structure doesn't "freeze" upfront the way it does in relational databases.

#The key reasons

1. No fixed tables and columns In MongoDB you don't need to describe a schema upfront. Documents in the same collection can have different sets of fields - the database won't complain if one document has a phone field and another doesn't.
2. Easy to change the data structure Adding a new field doesn't require migrating the whole collection. You just start writing the new field into new documents, while the old ones stay as they are.
3. Support for nested structures The document model lets you store arrays and nested objects right inside the document. That removes the need for complex JOINs and rigid relationships.
4. Different data versions can coexist The application can be updated gradually: some data is already in the new structure, some is still in the old one, and that doesn't break the system.

The essence of it: MongoDB gives you freedom, data can evolve alongside the application without constant rigid migrations and schema changes.