Which of the following would not be an express contract?

• Stella signs a contract to pay X dollars every Y months to Debra for rent.
• The milkman drops a free bottle off at Eleanor's doorstep each weekend, as he has done for the past 13 years.
• George agrees orally to purchase a painting from Sue.
• Timmy finds a note on the fridge from his mother saying she will pay him $20 to mow the lawn.