Ammo

Ammo was a type of collectible item released during the Treehouse of Horror XXV Event. It was used to shoot down the Rigellian UFOs that were seen flying around a Neighbor's Springfield. Ammo could be collected three different ways: it had a small chance of being dropped when tapping on Rigellians or Grem-Aliens, it could be bought in the store, or it had a small chance of being in the Treat Bags. The Ray Gun, even if owned from previous Events, would also spawn 1 Ammo every 8 hours.

Rigellian UFOs
Rigellian UFOs had a 25% chance of spawning in Neighbor's town, and it could only be shot down using Ammo.

Odds
When UFOs were shot down, they yielded prizes varying from the treats found in Treat Bags, to the Treat Bags themselves, to even a chance of getting Donuts.