GROSS (Get Rid Of Slimy girlS) is a club started by Calvin.
Originally, Calvin and Hobbes planned to have their clubhouse in the garage, but after the catastrophe of "moving" the car out of the garage (which ended with the car rolling into a ravine it was decided to have the clubhouse in Calvin's tree fort. In one comic, Calvin and Hobbes had an emergency meeting in the "Box of Secrecy," a cardboard box, because Susie had to stay at Calvin's home for an afternoon, much to Calvin's dismay.
Since Hobbes is capable of climbing into the tree fort without the rope ladder, he made up a password extolling the virtues of tigers that Calvin has to recite (and apparently, do a dance to also) in order for Hobbes to let down the rope ladder so Calvin can climb up. It is uncertain how long the password is, but there are at least eight verses to it.
Verse One: "Tigers are mean! Tigers are fierce! Tigers have teeth and claws that pierce!"
Verse Two: "Tigers is great! They can't be beat! If I was a tiger, that would be neat!"
Verse Six: "Tigers are nimble and light on thier toes. My respect for tigers continually grows."
Verse Seven: "Tigers are perfect, the E-Pit-O-Me of good looks and grace and quiet dignity."
Final Verse (Presumably): "Tigers are great! They're the toast of the town. Life's always better when tigers are around!".
"Tigers are awesome! Tigers are sweet! If you really don't think so, then go smell my feet!"
Password in Calvin and Hobbes 2
"The world is better without demons that spread around the city. The world would fear evil. Down with DarkKid Calvinater Richardo the III!"