Despite a thorough search, it is impossible to locate the source of this smell since it seeps into our reality from extra-dimensional space where the dimensions intersect. After successfully neutralising a localised greebling threat our furry aliens will sit motionless, relaying this information back to the mothership. Their whiskers and ears may twitch as the signals are sent and mothership responses received. One or two cats per household is generally sufficient to balance the greeblings. In cat-free zones, the greeblings run amok, the major effect of which seems to be a subtle but profound emotional malaise known as Sick Building Syndrome.
Some areas are more prone to greebling attack, perhaps due to localised gravity wells.
In these areas, humans are gently manipulated to adopt whole squadrons of cats to neutralise greebling incursions. Greeblings are not especially physically harmful to humans, but if allowed to build up to sufficient numbers they can cause annoying problems with household appliances, plumbing, electrical systems, car engines and computers. Greeblings have weak electrically charged fields, which may be either negatively or positively charged. This may be how greeblings determine gender, reproductive condition or even identify individual greeblings. It has been conjectured that dust bunnies are caused by the greeblings' electrical field though this hypothesis has now been rejected.