CritterTrail Bubble-Plugs are used to close opened accessory holes on the sides of CritterTrail homes. Bubble Plugs can also be attached to a 10 Fun-nels climbing Tube to close an opened accessory hole on the roof top of any CritterTrail home. Bubble Plugs are available in four colors and are sold two to a package - 2.25" Diameter (4 Pack).