Capsule

The Prison Eggs are large capsules, used by Doctor Eggman to hold captive animals, presumably so that they can later be turned into badniks. The Prison Eggs have appeared in various shapes, but are most often a metal container with a single switch on the top. Pressing this switch will either destroy or open the Egg, releasing the prisoners.

Prison Eggs appeared at the end of each Zone, following the final boss battle (usually against Doctor Eggman) in Sonic the Hedgehog (16-bit and 8-bit), Sonic the Hedgehog 2 (16-bit and Sonic the Hedgehog), Sonic the Hedgehog CD, Sonic the Hedgehog 3, Sonic & Knuckles, and Sonic Advance. They also appear at the end of some action stages in Sonic Adventure. Once the capsule is opened, the player has officially reached the end of the Zone and the scores are tallied.

In the Flying Battery Zone in Sonic & Knuckles, Eggman's floating fortress contains numerous Prison Eggs throughout the level. Some do contain animal friends, but others contain rings, badniks, or nothing at all. In some cases, the switch on top of the capsule may act as a spring. An Egg Prison at the end of the Zone's first act will turn into the act's sub-boss, Gapsule.