For the film Bleu Krzysztof Kieslowski needed a moment to show a character in her own world, focused on one single action, the action was allowing a sugar cube touch a cup of coffee until it turned fully brown, for this purpose he used an insert of the cube becoming brown. Simple enough, however, the question is, how long should this insert take? The answer is 5 seconds, if it were to be any longer it would become boring but any less and it wouldn’t be really noticeable for the audience. So his assistant had to check several cubes to find a suitable one.
You should see Kieslowski explaining this himself:
Bleu (1993) Krzysztof Kieslowski