One possible challenge to teach the learning pit strategies to grade 8 students could be a group project where they have to design and build a Rube Goldberg machine. A Rube Goldberg machine is a complex contraption that performs a simple task through a series of chain reactions.

Here’s how you can structure the challenge:

1. Divide the students into groups of 4-5.
2. Explain the concept of the learning pit and how it relates to problem-solving and critical thinking.
3. Introduce the challenge: Each group has to design and build a Rube Goldberg machine that can pop a balloon.
4. Provide them with a set of materials such as dominos, marbles, ramps, pulleys, string, etc. They can also bring in additional materials from home if needed.
5. Give them a specific time frame to plan and design their machines. Encourage them to think about cause and effect, and how each component will trigger the next.
6. Once the planning phase is over, allow them to start building their machines.
7. As they encounter obstacles or challenges during the construction process, encourage them to reflect on their strategies and use the learning pit strategies to overcome the difficulties.
8. After completion, have each group present their Rube Goldberg machine to the class, explaining the different components and how they overcame challenges along the way.
9. Facilitate a class discussion on the learning pit strategies used by each group and how they contributed to their success.
10. Finally, have a class vote to determine the most creative and effective Rube Goldberg machine.

This challenge will not only engage the students in hands-on learning but also provide them with an opportunity to apply the learning pit strategies in a practical context.