Your TV can see through walls

A user experimented with a TV signal and observed that walking between the receiver and TV station caused a disturbance in the signal, which was visible as a waterfall plot with gaps between the stripes. The user calculated that the speed of the movement (in this case, walking) could be estimated by measuring the gap between the stripes and the update rate of the waterfall plot. This technique can be applied to other wideband radio signals like WiFi or LTE, but the results may be less clear due to a messier waterfall plot.