MatchBox cars as a switch
I tied conductive thread to the metal undercarriages. When they crash into each other it closes a switch, which the arduino detects and demonstrates by blinking an LED (orange, foreground).
This was an early stage of testing for my Crash Photography project:
An arduino detects the collision of the matchbox cars and flashes a light at that exact moment in front of the open camera shutter to capture an exposure.
Full write-up here: Crash Photography with Arduino and MatchBox Cars