The discussion section is probably the most difficult and challenging to write because you have to think carefully about the specific results you obtained in your experiment, relate them to the aim, interpret them and generalise from them. In this way you relate your own results to the store of scientific knowledge.

Some points to remember:

Staging If you have a number of results to discuss, the sequence of stages may need to be repeated and it is helpful to give each sequence its own sub-heading. For this kind of discussion, you may need to begin with a more general stage that gives brief background information, reminding your reader of the aim of your experiment, the method employed, relevant previous history, or theory.
Argument Your discussion is an argument about how you see your results. Use the questions and the stages mentioned here to help you to develop your argument in a logical way. Remember that not all of the questions or the stages will be relevant to every experiment.
Content Finding the balance between what to put in the results section and what to put in the discussion can also be a problem for student writers. Remember that your results section should not contain explanations of the significance of the results.  This belongs in the discussion. Sometimes your discussion section will also include your conclusion and in this case your overall heading will be ‘Discussion and Conclusion’.

Tim highlights the problems students have with understanding the fundamental concepts behind the experiment (22s):

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