Question

Why would you choose subjects like "My Hobby" or "My Pet" for a five-minute speech?

because I would have little or no knowledge about topics like exponential notation, semantics, etc.
because talking about a pet or a hobby can be done in five minutes
because topics that relate to my own life are well enough known that I can speak about them with confidence

Answer 1

Choose 2 and 3.

- 2 (talking about a pet or hobby can be done in five minutes): these topics have a naturally limited, manageable scope so you can organize a clear intro–body–conclusion and fill five minutes without rambling or running out of things to say.
- 3 (topics that relate to my own life are well enough known that I can speak about them with confidence): personal familiarity gives you concrete details, stories, and emotions to draw on, which improves fluency, reduces preparation time, and helps engage the audience.

Why 1 is not a good reason: avoiding difficult subjects because you “have little or no knowledge” isn’t a strong rationale — you can simplify technical topics or learn enough to present them, and choosing a topic should be based on suitability for the time and your ability to speak confidently rather than just what you don’t know.