Isolation between programs usually means no network and no shared files. A covert channel works around that by encoding bits in a side effect both can observe. One program, for example, changes what sits in a shared processor cache, and the other measures how fast its own data loads and reads the difference as a 0 or a 1. Such channels are typically slow, but they show that isolation on shared hardware is never absolute. Security teams study them to judge how well sandboxes contain software, including AI agents.