Some concepts are so simple to explain that they do not need a (visual) metaphor. If you don't need one, don't use one. They will sound forced, and actually makes the message of your presentation more difficult to understand.
Here are situations where I do rely on metaphors:
- Make size comparisons that are hard to imagine
- When there are lot of different things that need to be covered. You can set up the metaphor at the beginning of the presentation, and continue to build on it throughout the deck.
- When it is possible to compare yourself to a winning business idea that revolutionized another market, preferably a long time ago.
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