A two-phone X-ray diffraction lab. One phone is the crystal — tilt it under the beam. The other is the detector — sweep it through the air to catch the scatter spots and name the mineral.
Both phones must enter the same room code. iOS will ask for Motion & Orientation and Camera access — tap allow. Works best in Safari over Wi-Fi in the same room. A single-phone version explains the physics.
Testing alone? Open a stand-in bot on a laptop with the same room code to play the other side.