DESC: The CritterTrail Snap-on Comfort Wheels is extra silent and has a solid running surface to provide a safe exercise source for your little friend. The wheel features smooth threads that allow for a surefooted grip while creating a comfortable running surface. This 6.75" wheel is sized perfectly for hamsters and gerbils. The enclosed wheel attaches directly to the side of any CritterTrail Home and also attaches directly to CritterTrail Fun-nel Tubes for added exercise fun.