system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do heat-seeking missiles work? do they work exactly like in the movies?
gpt
Early ones were really simplistic and just pointed the missile to the hottest thing it sensor could detect, be that a planes exhaust, the sun or flares (decoys). They had limited steering ability and only worked when shot at a plane from behind it (called "rear-aspect"), and even then weren't that reliable.  

Modern ones are much much more sophisticated. They have high-resolution infrared cameras, can detect and track planes from all angles, ignore flares, plot efficient intercept courses, are much more manouvrable and fully integrated into the planes targeting systems.   

A modern AIM-9X for example can be given targeting data from the planes radar or helmet mounted sight prior to launch and can track a target up to 90° to the sides (of boresight), allowing pilots to shoot at targets without having to point their own planes nose even close to it.