diff --git a/README.md b/README.md
index df43b4c..66c2de9 100644
--- a/README.md
+++ b/README.md
@@ -18,7 +18,7 @@ traders_copilot_mzza_25 provides functionalities for generating trading signals,
 
 - generate_signals: a function that identify buy/sell signals based on price trends.
 - plot_signals: function that visualizes the timestamp and input values.
-- fetch_market_data: a function that fetches market data from the internet.
+- simulate_market_data: a function that simulate stock market data.
 - buy_stock: a function that buy stock and update user account.
 - sell_stock: a function that sells stock and update user account.
 
diff --git a/src/traders_copilot_mzza_25/buy_stock.py b/src/traders_copilot_mzza_25/buy_stock.py
index e69de29..0c33ce6 100644
--- a/src/traders_copilot_mzza_25/buy_stock.py
+++ b/src/traders_copilot_mzza_25/buy_stock.py
@@ -0,0 +1,31 @@
+def buy_stock(account, quantity, price):
+    """Execute a stock purchase and update the user's account.
+
+    This function processes the purchase of a specific quantity of a stock at a given price.
+    It updates the user's account balance and increases the number of shares held for
+    the specified stock.
+
+    Parameters
+    ----------
+    account : dict
+        A dictionary representing the user's account. Expected keys include:
+        - 'balance': float, the current cash balance in the account.
+        - 'stocks': int, the number of shares held.
+    quantity : int
+        The number of shares to buy. Must be a positive integer.
+    price : float
+        The price per share at which the stock is purchased.
+
+    Returns
+    -------
+    dict
+        The updated account dictionary reflecting the new balance and stock holdings.
+
+    Raises
+    ------
+    ValueError
+        If the account balance is insufficient to complete the purchase.
+
+    Examples
+    --------
+    """
diff --git a/src/traders_copilot_mzza_25/fetch_market_data.py b/src/traders_copilot_mzza_25/fetch_market_data.py
deleted file mode 100644
index 1d685eb..0000000
--- a/src/traders_copilot_mzza_25/fetch_market_data.py
+++ /dev/null
@@ -1,13 +0,0 @@
-def fetch_market_data(url):
-    """Fetches market data from the internet.
-
-
-    Parameters:
-    ----------
-    url: A link to the website that contains market dataset.
-
-    Returns:
-    ----------
-    pd.DataFrame
-        The dataframe that contains columns for market data.
-    """
\ No newline at end of file
diff --git a/src/traders_copilot_mzza_25/sell_stock.py b/src/traders_copilot_mzza_25/sell_stock.py
index 8cce148..cc71ad0 100644
--- a/src/traders_copilot_mzza_25/sell_stock.py
+++ b/src/traders_copilot_mzza_25/sell_stock.py
@@ -1,4 +1,4 @@
-def sell_stock(account, stock_symbol, quantity, price):
+def sell_stock(account, quantity, price):
     """Execute a stock sale and update the user's account.
 
     This function processes the sale of a specific quantity of a stock at a given price.
@@ -10,9 +10,7 @@ def sell_stock(account, stock_symbol, quantity, price):
     account : dict
         A dictionary representing the user's account. Expected keys include:
         - 'balance': float, the current cash balance in the account.
-        - 'stocks': dict, mapping stock symbols to the number of shares held.
-    stock_symbol : str
-        The symbol of the stock being sold (e.g., 'AAPL', 'GOOG').
+        - 'stocks': int, the number of shares held.
     quantity : int
         The number of shares to sell. Must not exceed the number of shares currently held.
     price : float
@@ -30,10 +28,4 @@ def sell_stock(account, stock_symbol, quantity, price):
 
     Examples
     --------
-    >>> account = {'balance': 1000.0, 'stocks': {'AAPL': 10}}
-    >>> sell(account, 'AAPL', 5, 150.0)
-    {'balance': 1750.0, 'stocks': {'AAPL': 5}}
-
-    >>> sell(account, 'AAPL', 10, 150.0)
-    ValueError: Not enough shares to sell.
     """
diff --git a/src/traders_copilot_mzza_25/simulate_market_data.py b/src/traders_copilot_mzza_25/simulate_market_data.py
new file mode 100644
index 0000000..50e5537
--- /dev/null
+++ b/src/traders_copilot_mzza_25/simulate_market_data.py
@@ -0,0 +1,14 @@
+def simulate_market_data(start_date):
+    """Simulate stock market data that contains date and price columns
+    from start date until today.
+
+
+    Parameters:
+    ----------
+    start_date: Start date of the simulation (YYYY-MM-DD).
+
+    Returns:
+    ----------
+    pd.DataFrame
+        The dataframe that contains columns for stock market data.
+    """
\ No newline at end of file