The Dog Zeroth Law.

[Gandahar]

A dog will always expand to fit any length of sofa you put them on

[R E S T E C P]

True... On the rare occasion our Labrador has been allowed on the bed, he has somehow managed to lie in a star shape taking up the entire bed and pushing the human occupants off. It's super-king sized

[dnb]

Dogs (and children) are best described as having most of the properties of an ideal gas. They expand to take up their entire enclosure.