The original rules are designed for a fast-paced adventure style of gaming, but also help to represent the sophisticated hierarchy of rat culture, assuming you want to call anything rat-related “sophisticated”. They are centered around the four traits Strong, Quick, Social and Clever, as well as some accordlingly straight-forward-named Talents and Tricks like “Out of the Way” or “Wound Licking”. In the end, you roll a number of six-sided dice dependent on your traits and add up the two highest scores. If you beat a difficulty, congratulations! You live to see another day, or whatever your goal was. The numbers involved are small enough to be appropriate for schoolboys and -girls (the throat-biting and extensive procreating, on the other hand, might not). There is also a dirty and dangerous combat system without extensive tactical rules, but with extensive wrestling around on the floor and biting things. If you bite things successfully, or do whatever fits you horde best, you can also collect songs, tales other rats might have heard about you. Which is just great if you want them to do something for you, or just impress your rat-friends. If you are especially brave, cunning or stupid (but in a martyr-like way), you might get a new name fitting your deed. Some rats have three names or more. And yes, they will use every single one of them. Always.
Besides the original rules, Packs! can be played with the renowned Savage Worlds system as well.