This paper model is a Rollermine, an electrically charged, spherical device slightly larger than a soccer ball that reacts to movement within a sizable radius, based on the Half Life 2 sci-fi video game, the papercraft is created by Chamoo232, and shared by The Military Pirate.
Rollermines are used as offensive and defensive traps by the Combine. As the name implies, they roll towards enemies and electrocute them via close-range electric shocks. Extensively used along the coastal roads, they are deployed from the air by Combine Dropships, in groups of two to four. Upon landing, the mines will remain inert or, if they land on softer terrain like soil or gravel, they will burrow themselves into the ground, remaining almost invisible. When an enemy target is detected, the roller mine will pop out of hiding and roll towards it in a relatively straight line.
The mines can magnetically cling to a target vehicle and interfere with the steering, forcing the driver to either stop or drive off the road. Besides the damage inflicted by any resulting car wreck, attached Rollermines do not directly injure the occupants of a vehicle. For this reason, Rollermines are usually deployed in tandem with Combine soldiers, who ambush the stranded individuals. Attached mines cannot be removed except through excessive force, chiefly the Gravity Gun; touching them will merely cause electrocution. They are also deployed in buildings and structures, taking any intruder by surprise. Rollermines can be destroyed with explosive devices, or alternatively may be picked up with the Gravity Gun and launched out of the way. Complete submersion in water also causes them to short-circuit and explode. Additionally, one can elude Rollermines by outrunning them, as they cease chasing once a target is too far away. If Rollermines are caught in an explosion, they bounce up and detonate.
Alyx was at one point able to capture and disarm a Rollermine, which she then gave to her "pet" Dog to play with, calling it a ball. Alyx had Dog use the "ball" to play catch with Gordon Freeman so Freeman could get practice using the Gravity Gun.
In Episode One, Alyx reveals that she can use her EMP tool to temporarily reprogram a Rollermine's targeting system, making it "friendly". Once reprogrammed, Rollermines will emit an orange light instead of their signature blue and will follow the player slowly, attacking any Combine units it sees. Reprogramming Rollermines makes them unstable, however, and they will invariably emit red light and explode a few minutes later. [Source: wikia]