This research is usually done in the designing stage not at the start although depending on your design brief you may need to do it earlier. This is all based on the specific FUNCTIONALITY of the product and is often to do with more complex and technical aspects like mechanical or electrical aspects of your design

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For example, you might be designing a camping torch and one of your initial design's has included solar panels as the energy source. In order for you to include these, you will need to research how they work so that your design includes them in a way that will actually work.