|Art by Tom J. Manning|
"Empties are two hand-sized discs always 29 centimetres apart. If one is moved, the other will also move to hold its position. No force can move the discs closer or further apart.There is nothing between them, so they can, for example, be located on opposite sides of a wall." - Stalker RPG, p. 35
But sometimes the Empties have something inside them, such as a swirling gas which stays within the virtual canister formed by the two discs. These are known as Full Empties. While Empties are very common "finds" in a Zone, Stalkers can really strike it rich when they find a Full Empty.
Full Empties are rare.
Some have swirling colored gas inside; others, a liquid. They can also store something like grains or pellets. Still other Full Empties store discrete objects such as artifacts or creatures/phenomena found in the Zones, such as silver webs.
At least one Institute is studying the Empties to determine whether time "inside" an Empty passes at the same rate as everything outside one.
Are they Alien canteens? Food storage? Reliquaries? Fuel containers left behind as roadside trash after the Visit?
Perhaps they are bug jars for alien butterfly hunters.