There’s only one catch to this trick, and that’s getting close enough to a girl’s boobs that you can shine your phone’s flashlight right up against them – but if you’ve got that solved, you should be golden.
Why do fake boobs light up, you wonder? It’s because the silicone implants beneath the skin absorb/bounce the light from the phone around the implant, causing them to glow.
So now you know: if your girlfriend’s boobies ever start acting like real headlights, they’re indeed fake.