system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do reflexes...work?

Just the title, how does your body know what to do with these stimuli without the signals actually going past your brain?
gpt
Because your brain isn't everything - it's just one part of "the central nervous system", which includes the brain stem (this tends to do basic tasks like how to breathe and how to eat), the higher brain (which does most of the "human-y" bits like knowing how to do 2+2 and being angry about celebrities), and the spinal cord, which is both a big highway of information and a reflex processing centre. 

Reflex systems are a bit like wires in electronics. Imagine you have a computer, that does all the standard things a computer does. It also has lights on the box for style value, and it has a separate button you can press to change the light. That button doesn't have anything to do with the circuitry of the computer, it's just one switch on the case that has a wire connected to the LEDs. Flick the switch and the light turns on, switch it again and it turns off - and it'll do this even if the computer is broken, cos it's a really simple light switch system that's just part of the case. 

A reflex system is composed of three types of neurones - sensory neurones, which detect the stimulus; motor neurones, which tell muscles to contract; and relay neurones, which act as a bridge between the sensor and motor, and also something that allows the brain to override the signal if necessary. When the sensory neurone detects the stimulus (let's say the pain of touching a hot pan), it sends an electrical signal all the way up your arm and to its associated relay neurone in the spine. The relay neurone sees this message and as long as it's not also getting a message from the brain saying "don't do the reflex", it'll spontaneously send a new message into its associated motor neurone. The signal travels down the motor neuron all the way back down your arm, and at the end, it tells the muscles in your arm to pull back so that you stop touching the hot pan. 

There's almost no information processing actually going on here. Reflexes are pretty much completely automatic and they're pre-programmed to know what to do when they receive a signal.