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For obvious reasons you need to prove - first to yourself, later to investors or companies - that your idea works. For an invention that is a process or a business method, physical proof may not be possible or necessary. For an invention that is a substance (for example, a new kind of shampoo), samples to test may be sufficient.
But if your invention is a manufacturable product, you need to show it looking as close to a finished article as you can manage or afford. That usually means producing at least one and often a series of prototypes. This may be where your first serious costs begin, so you need to plan and control your prototyping activities.
For some inherently costly ideas you may need to seek funding for the prototype itself. In that case you must gather convincing evidence that your idea will work, and has the potential to make enough profit to justify the much greater level of risk.