Click the button to gain experience.
Game automatically saves so whenever you need to work on grinding and leveling up you can come back and it will be right where you left off, goodluck!
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No point to the leveling system at all, you could make unlockables (different backgrounds, themes, maybe background music) to at least make it 5% more interesting. Just plastering "YOLO" on a big red button doesn't make it cool.