There are six weights - two white, two red and two blue. Apart from the colours all the weights are visibly identical. In each coloured pair of weights one is light and one is heavy. And all three light weights are the same weight, as are the three heavy weights.
Can you you determine which weight in each coloured pair is the heavy weight using two weighings on a balance scales?
There are a few methods, but this one is quite elegant.
Firstly weigh a white weight and a blue weight against the other white weight and a red weight.
If this balances then the heavy white weight is with a light weight and vice versa. For the second weighing simply remove both white weights. This will reveal whether the blue or red weight is heavy, and the others can all be inferred from this.
If one side goes down then the light and heavy white weights are revealed, and red and blue weights are both light or both heavy. For the second weighing remove the white weights and weight the red weight against the other blue weight. This will reveal whether the original red and blue weights were both light or both heavy, and the others can be inferred from this.