diff --git a/README.md b/README.md
index 907a7a0..f50f0f7 100644
--- a/README.md
+++ b/README.md
@@ -203,6 +203,14 @@ fiable_db.find_one(id=1) # Default table
 # {"id": 1, "rev": 1, "data": {"name": "Miguel", "age": 41, "height": 189}}
 ```
 
+### Step 9: Save changes
+
+Save the database to a file.
+
+```python
+fiable_db.save(filename, data)
+```
+
 ### Other help functions
 
 #### Get all data
@@ -210,7 +218,7 @@ fiable_db.find_one(id=1) # Default table
 Get all data from the database.
 
 ```python
-get_database()
+fiable_db.get_database()
 ```
 
 ### Load file
@@ -218,15 +226,7 @@ get_database()
 Load a file into the database.
 
 ```python
-load(filename)
-```
-
-### Save file
-
-Save the database to a file.
-
-```python
-save(filename, data)
+fiable_db.load(filename)
 ```
 
 ## Implementations in other languages
diff --git a/fiable_db.py b/fiable_db.py
index 7bc2508..a3ad9b8 100644
--- a/fiable_db.py
+++ b/fiable_db.py
@@ -1,12 +1,14 @@
-from os import path
-import json
 from typing import Dict, Tuple, Union, Sequence, TypedDict
+import json
+from os import path
 
 # Variables
 FILE = "fiabledb.json"
-database = {}
+database = []
 
 # Type aliases
+
+
 class TypeData(TypedDict):
     id: int
     rev: int
@@ -15,13 +17,20 @@ class TypeData(TypedDict):
 
 Type_Data_List = Tuple[TypeData]
 Type_Add_Data = Union[Dict, Sequence[Dict]]
-Type_Add_Return = Union[Tuple[int, int, Dict], Tuple[Tuple[int, int, Dict]], None]
+Type_Add_Return = Union[Tuple[int, int, Dict],
+                        Tuple[Tuple[int, int, Dict]], None]
 Type_Update_Return = Union[Tuple[Tuple[int, int, Dict]]]
-
 Type_Delete_Return = Union[Tuple[Tuple[int, int, Dict]]]
 Type_Find_One_Return = TypeData
 Type_Find_All_Return = Tuple[Type_Find_One_Return]
 
+# Functions
+
+def get_next_id(table: str = "default") -> int:
+    """Get the next id for a table"""
+    global database
+    return database[-1]["id"] + 1 if database else 1
+
 
 def start(file_name: str = "") -> str:
     """Start the database
@@ -87,7 +96,7 @@ def get_database() -> Type_Data_List:
     return database
 
 
-def add(new_data: Type_Add_Data, table: str = "") -> Type_Add_Return:
+def add(new_data: Type_Add_Data, table: str = "default") -> Type_Add_Return:
     """Add data to the database
     Args:
         new_data (dict|list): The data to add
@@ -95,13 +104,17 @@ def add(new_data: Type_Add_Data, table: str = "") -> Type_Add_Return:
     Returns:
         dict[int, int, dict]|list[dict[int, int, dict]]: The data added
     """
+    global database
     if isinstance(new_data, dict):
-        return _add(new_data, table)
+        new_row = {"id": get_next_id(table), "rev": 1, "data": new_data}
+        database.append(new_row)
+        return new_row
     elif isinstance(new_data, list):
-        return [_add(entry, table) for entry in new_data]
+        for row in new_data:
+            new_row = {"id": get_next_id(table), "rev": 1, "data": row}
+            database.append(new_row)
     else:
         raise TypeError("new_data must be a dict or list")
-    print("Function not implemented yet")
 
 
 def update(
diff --git a/test/test_add.py b/test/test_add.py
new file mode 100644
index 0000000..46c1117
--- /dev/null
+++ b/test/test_add.py
@@ -0,0 +1,62 @@
+from fiable_db import add, get_database
+
+
+def test_add_one():
+    """Add one item to the database."""
+    add({"name": "John", "age": 42})
+    assert get_database() == [{"id": 1, "rev": 1, "data": {"name": "John", "age": 42}}]
+
+
+def test_add_two():
+    """Add two items to the database."""
+    add({"name": "John", "age": 12})
+    add({"name": "Jane", "age": 34})
+    print(get_database())
+    assert get_database() == [
+        {"id": 1, "rev": 1, "data": {"name": "John", "age": 42}},
+        {"id": 2, "rev": 1, "data": {"name": "John", "age": 12}},
+        {"id": 3, "rev": 1, "data": {"name": "Jane", "age": 34}},
+    ]
+
+
+def test_add_list():
+    """Add a list of items to the database."""
+    add(
+        [
+            {"name": "John", "age": 12},
+            {"name": "Jane", "age": 34},
+        ]
+    )
+    assert get_database() == [
+        {"id": 1, "rev": 1, "data": {"name": "John", "age": 42}},
+        {"id": 2, "rev": 1, "data": {"name": "John", "age": 12}},
+        {"id": 3, "rev": 1, "data": {"name": "Jane", "age": 34}},
+        {"id": 4, "rev": 1, "data": {"name": "John", "age": 12}},
+        {"id": 5, "rev": 1, "data": {"name": "Jane", "age": 34}},
+    ]
+
+
+def test_add_list_with_one():
+    """Add a list with one item to the database."""
+    add([{"name": "John", "age": 42}])
+    assert get_database() == [
+        {"id": 1, "rev": 1, "data": {"name": "John", "age": 42}},
+        {"id": 2, "rev": 1, "data": {"name": "John", "age": 12}},
+        {"id": 3, "rev": 1, "data": {"name": "Jane", "age": 34}},
+        {"id": 4, "rev": 1, "data": {"name": "John", "age": 12}},
+        {"id": 5, "rev": 1, "data": {"name": "Jane", "age": 34}},
+        {"id": 6, "rev": 1, "data": {"name": "John", "age": 42}},
+    ]
+
+
+def test_add_empty():
+    """Add an empty list to the database."""
+    add([])
+    assert get_database() == [
+        {"id": 1, "rev": 1, "data": {"name": "John", "age": 42}},
+        {"id": 2, "rev": 1, "data": {"name": "John", "age": 12}},
+        {"id": 3, "rev": 1, "data": {"name": "Jane", "age": 34}},
+        {"id": 4, "rev": 1, "data": {"name": "John", "age": 12}},
+        {"id": 5, "rev": 1, "data": {"name": "Jane", "age": 34}},
+        {"id": 6, "rev": 1, "data": {"name": "John", "age": 42}},
+    ]