Ironically, the explanation for this illusion is that we expect the largest case to be heaviest and the smallest to be lightest. Our motor system makes allowance for this expectation so when the object requires less effort to lift than we expected, our brain perceives the case as being lighter than it really is.
The reverse happens with the smallest cases — we expect it to be much easier to lift than it is, so our brain perceives it as heavier than it is.
You’ll need:
- 1 large suitcase
- 1 medium suitcase
- 1 briefcase
- A set of scales
- Several books
The preparation
Fill the briefcase with books and weigh it. Now place enough books (or other objects) in the other two cases so that all three cases weigh the same.
The activity
Ask a friend to lift each case in turn and tell you which feels heaviest. People invariably say that the briefcase is heaviest and find it hard to believe the truth when you put them on the scales.