Gapsule (ガプセル) is the second mini-boss of Sonic & Knuckles, fought at the end of Flying Battery Zone Act 1.
Gapsule is one of the more bizarre boss battles of the Sonic series, with the player battling a trap that looks like an animal Capsule. It also takes some thinking to figure out how to damage this boss, as it is immune to all direct attacks. Every so often, Gapsule sends one of its spiked balls (which at other times flail about in the air on chains) homing in on the player. To defeat it, the player must stand on its button and wait for its eyes to blink; that's the signal that it's about to send a ball flying toward the player. Then, the player must quickly run off the button, being careful not to touch a spiked ball. The spiked ball will hit the button, damaging Gapsule. This process must be repeated for a total of six times in order to destroy Gapsule.