What life hack is so simple yet so effective, you're shocked more people don't know about it?

Not really a life hack but: folding bicycles are so, so convenient for everyday use (fits in every elevator, fits under your desk at work, fits in public transportation even when it's crowded, etc)

I really don't know how they are not a lot more common; you only need a mountain bike if you actually ride on mountain trails often enough. Even non-folding city bicycles are way more comfortable for everyday usage (higher handlebar position and cushiony saddle <3)