Soap dispenser
How does it know your hand is there?
It shines light you can't see and counts how much bounces back. Put a hand under it, change the glove, then try a white card with no hand at all.
Your hand
Fool it
Senses
Rule
Does
For grown-ups: what's really inside
Most touch-free dispensers have an infrared LED and a light sensor side by side. The LED pulses light people can't see; anything under the spout bounces some back. If the sensor gets more than a set amount, a small pump runs once.
Dark surfaces bounce back less infrared, so a sensor tuned only on light-colored hands can miss darker ones. Real dispensers have shipped with exactly that problem. AI systems fail the same way: they only work for the cases they were tested on, so test with everyone who will use it. Simplified: real sensors also filter out room light and need the reading to last a moment.