Curious....

How do automatic flushing toilets know when to flush? Thinking back, I remember that there seems to be a sensor behind the seat so when a shadow or some object goes in front of it, viola, a flush. But then I was thinking that, how does it know not to flush when you sit down? And it often starts flushing once I have zipped up and getting ready to head out of the stall. Perhaps a sensor on the door? Very curious.