system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5 why does the flush sensor on a touchless toilet go off and splash my butt at the slightest movement while I’m sitting down, but the touchless sensor on a sink won’t turn on unless I’m in JUST the right spot?
gpt
The flush sensor on a toilet faces toward the front of the stall, and the sensor spreads out in a V. It expects to sense 3-4 feet away. It works when it senses a bounce back (which could be anywhere bouncing around the walls as you move).

The flush sensor on a sink is facing down into a curved reflective surface which is constantly bouncing back the signal. It works when it _doesn't_ sense a bounce back, so you have to block the entire sensor.