A 5×3, 10-line slot where the book symbol acts as both wild and scatter. 3 books trigger 10 free spins, and one "expanding symbol" is chosen at random.

During free spins, when the expanding symbol meets the win requirement, it expands to fill its reels and pays even on lines where the symbols aren't adjacent. If the high-paying symbol (the explorer) is the expanding symbol, you can see the whole screen fill up.

The rules are simple, but results vary hugely depending on which symbol expands. Countless book slots have since followed this structure.