What is the most interesting puzzle the monkeys have solved when you have worked with them?

Question: What is the most interesting puzzle the monkeys have solved when you have worked with them?
- Keywords:
  discovery,
  experiment,
  preference
Asked by bakerboy2002 to Lewis on 9 Mar 2014.
- Lewis Dean answered on 9 Mar 2014:
  
  There was a really complicated puzzle that we set them (it doesn’t sound that complicated, but it was – trust me!), where they had to slide a door open to get food. However, if they only slide the door open, they just got some carrot, if they pushed some buttons and turned some dials, they got better food.
  
  Only a few of the chimps and capuchin monkeys learned how to do it, most just kept getting carrot.

Question: What is the most interesting puzzle the monkeys have solved when you have worked with them?