system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: how do smart watches count your steps?

It's on your arm not your leg so how does it actually know how many steps you're taking? And do things like how fast you walk and how long your stride is make a difference?
gpt
By using the gyro mechanism inside to simulate the old style pedometers which used an arm and a spring to bounce with your walking movement. The detected movement would be counted as "steps". 

Modern devices also use GPS to determine if you are actually moving and how fast so that it can more accurately track walking and not give out free "steps" when you are exercising in place, riding a bicycle or driving in a car.