Step into a lift and your four bars quietly collapse to zero. The building is not being cheap with signal. What just happened is nearly two-hundred-year-old physics, and it has a name: a Faraday cage.
Your phone talks to a tower using radio waves, which are really just electric and magnetic fields wobbling their way through the air. Now surround those waves with a box made of metal. Metal is full of loose electrons, and the instant a wave arrives, those electrons shuffle around to cancel its field out. The wave gets soaked up and bounced back at the wall, and mostly never reaches the inside. A lift is basically a metal room that eats radio for lunch.
Michael Faraday showed this off in the 1830s by lining a room with metal foil and sitting inside it calmly while the whole thing got zapped. Nothing reached him. Same reason a car is one of the safer places to be in a lightning storm: the metal shell carries the charge around the outside and past you. The rubber tyres usually get the credit, but they are not the ones doing the work.
It is not a perfect cage, which is the fun part. A lift has a door seam, cables, little gaps, so a bit of signal leaks through, and that is your one flickering bar that appears and vanishes. The microwave in your kitchen runs the same idea in reverse: that speckled mesh on the door is a Faraday cage built to keep the microwaves trapped inside and out of the room.
I like this one because it is a clean reminder that signal does not reach everywhere. We build apps as if everyone has a spotless connection at all times, and a large part of the world simply does not. The software I find most worth building is the kind that keeps working when the bars hit zero.