Answer: Look through walls to count people.
With just one Wi-Fi router placed outside of a room, researchers at the University of California, Santa Barbara, were able to count how many people were walking around in a room with a margin of error of just two.
The setup uses a transmitter and a receiver to measure the signal strength and attenuation of Wi-Fi in the room. Since moving objects basically absorb Wi-Fi signals, the system was able to use that to note how many moving people there were.
TechCrunch reports that the system can be set up using off-the-shelf components and requires a calibration phase before use that can be done anywhere. In its current form, it can’t tell you where in the room the people are, and it can only count up to 20 people at a time.