Monster problem for working memory

The following problem1 demonstrates the impact of working memory limitations on processing information. Although there is no difficult conceptual thinking required to solve the problem, it has been reported that most university students require about 30 minutes to find the solution.

Three monsters, one small, one medium and one large, were each holding a globe. The globe came in three sizes only – small, medium and large, and each globe could be expanded or shrunk repeatedly to any one of these sizes but to no other size.

The small monster was holding the medium globe, the medium monster was holding the large globe, and the large monster was holding the small globe. They could change the size of the globes according to the following rules:

  1. Only one globe could be changed at a time.
  2. When two globes are the same size, only the globe held by the larger monster may be changed.
  3. A globe must not be changed to the same size as the globe of a larger monster.

What sequence of changes would allow the monsters to hold globes proportional to their size?

Although each globe changing rule is easy enough to understand, the problem is difficult because it is difficult to hold the rules in working memory. The original creators of the problem (Kotovsky, Hayes and Simon – 1985) found memorizing the rules of the problem to the point where they could be repeated effortlessly made the problem much easier to solve.

While it is easy to assume that a student can’t solve a problem because he/she doesn’t understand what needs to be done, the monster problem indicates that working memory limitations are just as likely to be the cause of the difficulty.

1.  from  “Instructional design for technology” by John Sweller (1999).

see A Deep Understanding of Memory


  1. John W says:

    is using my fingers to help visualise cheating?

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